From 62d0697f0e879d894d23b1bb6e76c32be4924c0b Mon Sep 17 00:00:00 2001
From: Oleh Omelchenko <ptrvtch@protonmail.com>
Date: Wed, 10 Jun 2026 17:10:18 +0300
Subject: [PATCH] Chart builder: data-aware defaults, one-click hint fixes,
 canvas preview, fullscreen modal

---
 docs/IMPLEMENTATION-PLAN.md                   |  12 +-
 docs/architecture/03-modal-system.md          |  11 +-
 .../05-rendering-theming-preview.md           |  17 +
 docs/architecture/06-type-inference.md        |   6 +-
 .../10-interaction-and-feedback.md            |  12 +
 docs/chart-builder-enhancement-scope.md       | 353 ++++++++++++++++++
 docs/chart-builder-research.md                |   9 +-
 docs/lyra-review.md                           |   6 +
 docs/spec/06-chart-builder.md                 |   6 +-
 .../components/ChartBuilderModal.module.css   |  63 +++-
 src/app/components/ChartBuilderModal.test.tsx |  96 ++++-
 src/app/components/ChartBuilderModal.tsx      | 139 ++++++-
 src/app/components/ModalShell.module.css      |   8 +
 src/app/components/ModalShell.tsx             |  16 +-
 src/app/modals/modal-registry.ts              |   9 +
 src/app/services/chart-renderer.ts            |  74 +++-
 src/app/stores/ChartBuilderStore.test.ts      |  13 +
 src/app/stores/ChartBuilderStore.ts           | Bin 10536 -> 10859 bytes
 src/core/chart-builder.test.ts                | 151 +++++++-
 src/core/chart-builder.ts                     | 205 ++++++++--
 20 files changed, 1133 insertions(+), 73 deletions(-)
 create mode 100644 docs/chart-builder-enhancement-scope.md

diff --git a/docs/IMPLEMENTATION-PLAN.md b/docs/IMPLEMENTATION-PLAN.md
index 25a4efc..5e4088a 100644
--- a/docs/IMPLEMENTATION-PLAN.md
+++ b/docs/IMPLEMENTATION-PLAN.md
@@ -289,10 +289,14 @@ the reference.
 
 **Goal:** no-JSON chart composition from a dataset → a new snippet.
 
-> **Enhancement backlog** beyond the Tier-B floor (aggregation, binning, stacking,
-> temporal granularity, sort/orientation, cardinality-based warnings, Tier C
-> intent-first) lives in [`docs/chart-builder-research.md`](chart-builder-research.md) §8
-> — its single home, so these stop living in chat.
+> **Enhancement push (post-M4).** The forward plan now lives in
+> [`docs/chart-builder-enhancement-scope.md`](chart-builder-enhancement-scope.md) — it merges
+> the Tier-B backlog ([`chart-builder-research.md`](chart-builder-research.md) §8) with the
+> Lyra interaction review ([`lyra-review.md`](lyra-review.md)) and sets a **Tier-C** target.
+> Shipped beyond the Tier-B floor so far: per-channel aggregate/bin/`timeUnit`, sort/stack;
+> **actionable hints** (one-click warning fixes); and a builder UX/perf batch (near-fullscreen
+> modal, canvas preview + canvas max-dimension guard, data-aware default pre-population). See
+> the scope doc §4 for the sequenced plan and current status.
 
 **Core**
 
diff --git a/docs/architecture/03-modal-system.md b/docs/architecture/03-modal-system.md
index 77446d0..8f3d4a2 100644
--- a/docs/architecture/03-modal-system.md
+++ b/docs/architecture/03-modal-system.md
@@ -11,7 +11,9 @@ authoritative architecture for adding, opening, closing, and rendering modals.
 - **At most one modal open at a time** (mandated by the product spec). Opening a
   modal closes any other; the two never overlap.
 - **Uniform dismissal**: close button, `Escape`, or backdrop click — never a
-  click inside the body.
+  click inside the body. A modal holding in-progress work can opt out of the
+  **backdrop** click (`dismissOnBackdrop: false`) so a stray click can't discard it
+  (the Chart Builder does); close button and `Escape` still dismiss.
 - **Accessible by default**: focus moves into the modal on open and returns to
   the trigger on close.
 - **Unsaved-change safety** for editing modals, with an explicit opt-out for
@@ -492,6 +494,13 @@ export function useFocusTrap<T extends HTMLElement = HTMLDivElement>(
 - Don't dismiss on clicks inside the body, and don't let Escape fire when no
   modal is open (the handler only exists while a modal renders).
 
+**Sizing & backdrop opt-out.** The shell picks a **size tier** by modal: a small form
+(Extract), a large two-pane manager (Datasets), or a near-fullscreen **work surface**
+(Chart Builder — a config pane plus a chart that wants room). The two larger tiers have a
+definite height so their inner panes scroll **internally** rather than the modal growing
+past the viewport. A modal opts a backdrop click out of dismissal with the registry's
+`dismissOnBackdrop: false` (above).
+
 ---
 
 ## Confirmation & alert dialogs
diff --git a/docs/architecture/05-rendering-theming-preview.md b/docs/architecture/05-rendering-theming-preview.md
index 80176ef..240e29f 100644
--- a/docs/architecture/05-rendering-theming-preview.md
+++ b/docs/architecture/05-rendering-theming-preview.md
@@ -101,6 +101,23 @@ async function rerender(node: HTMLElement, spec: TopLevelSpec, config: Config) {
 
 - **Do** pass `actions: false`. Astrolabe owns its own export/copy affordances;
   the library's overlay menu does not belong on the preview.
+- **SVG is the default renderer, canvas is an opt-in for many-mark previews.** SVG is
+  crisp/inspectable/copyable and stays the default for the editor's LivePreview. But an
+  SVG chart renders one DOM node per mark, so a many-mark chart (e.g. the Chart Builder's
+  default one-bar-per-row on a 10k-row dataset) costs **seconds** of main-thread
+  layout/paint per render (measured ~6.5s on 9994 rows; the chart paints _after_ it first
+  appears, freezing the tab). The **Chart Builder preview** therefore passes
+  `renderSpec(…, { renderer: 'canvas' })` — canvas is a single node and paints in
+  milliseconds. The raster trade-off is invisible for an ephemeral preview, and image
+  export (`view.toImageURL`) is renderer-agnostic.
+- **Canvas has a hard max dimension; SVG doesn't.** A canvas larger than the browser's
+  limit (~32k px/side, less on Safari) fails to allocate and draws _nothing_ — silently.
+  So for canvas, `renderSpec` first runs a headless (`'none'`) layout probe, reads the
+  resolved height, and throws `ChartTooLargeError(heightPx, limitPx)` when it exceeds
+  `MAX_CANVAS_PX ÷ devicePixelRatio`, so the caller can show the real cause. This is a
+  **render-size** limit (the chart is physically too big), distinct from the readability
+  cardinality warnings — don't conflate them. Only an _unbounded_ axis overflows: a
+  `width: 'container'` axis is bounded, so it's the deleted (natural-height) axis to watch.
 - **Do** call `view.finalize()` on every previous view before rendering a new
   one, and on component unmount.
 - **Do** keep exactly one live view per preview node.
diff --git a/docs/architecture/06-type-inference.md b/docs/architecture/06-type-inference.md
index 43b2495..98f8de1 100644
--- a/docs/architecture/06-type-inference.md
+++ b/docs/architecture/06-type-inference.md
@@ -301,8 +301,10 @@ trivially testable. The caller passes `null` for URL and non-tabular datasets.
 ### 3.3 Column stats: cardinality + numeric extent
 
 Alongside the display type, each column carries the two data-shape signals the
-**Chart Builder** needs for its data-aware Tier-B hints (spec §06; see
-`chart-builder.ts` `builderWarnings`):
+**Chart Builder** needs for its data-aware Tier-B hints (`chart-builder.ts`
+`builderWarnings`) **and** for its default pre-population (`smartDefaultEncodings`
+prefers a low-cardinality category over a high-cardinality key, so the builder never
+opens on a degenerate chart; spec §06):
 
 - **`distinct`** — the count of distinct non-empty values **in the sample**,
   counted only up to `DISTINCT_CAP` (50). Past the cap the exact number stops
diff --git a/docs/architecture/10-interaction-and-feedback.md b/docs/architecture/10-interaction-and-feedback.md
index 0c3da18..b1690cf 100644
--- a/docs/architecture/10-interaction-and-feedback.md
+++ b/docs/architecture/10-interaction-and-feedback.md
@@ -167,6 +167,18 @@ lives in [04 · Routing & Global Events](04-routing-and-events.md).
   uses `alert`/`status` roles by severity. Don't invent keyboard models; adopt the
   documented one.
 
+**Resolved — a control that removes its own container.** When activating a control deletes
+the element it lives in (e.g. a Chart Builder guidance hint's one-click **fix** button —
+the hint re-derives away once applied), focus must not fall to `<body>`. The rule (council:
+Carbon _Actionable notification_ + APG _Alert_): **announce the change politely and move
+focus to a stable neighbour.** Concretely, the builder writes "Applied: `<label>`." to a
+visually-hidden `role="status" aria-live="polite"` node and moves focus to the guidance
+region if any hints remain, else the surrounding pane (`tabIndex={-1}` anchors, focused only
+programmatically — no visible ring). Advisory hints themselves don't _grab_ focus (APG: an
+alert "must not affect keyboard focus"); the fix's remedy lives in a **low-emphasis ghost
+button** beside the advice (Carbon: inline actionable → ghost button, wraps under the body
+on narrow widths), an _offer_, never a forced change.
+
 **Resolved — pane resize handle (window splitter).** A `ResizeHandle` is a focusable
 `role="separator"` that **reports the controlled pane's size**, per APG → Window Splitter:
 
diff --git a/docs/chart-builder-enhancement-scope.md b/docs/chart-builder-enhancement-scope.md
new file mode 100644
index 0000000..3c22067
--- /dev/null
+++ b/docs/chart-builder-enhancement-scope.md
@@ -0,0 +1,353 @@
+# Chart Builder — Enhancement Scope
+
+> **Status:** scope consolidated 2026-06-10. This is the **single forward-looking home**
+> for chart-builder enhancement work — it merges the research backlog from
+> [`chart-builder-research.md`](./chart-builder-research.md) §8 (the M4 decision and its
+> deferred items) with the interaction ideas from [`lyra-review.md`](./lyra-review.md) §5,
+> read against the current spec ([`spec/06-chart-builder.md`](./spec/06-chart-builder.md))
+> and the shipped code (`src/core/chart-builder.ts`).
+>
+> **Goal (the brief):** a **rapid, intuitive GUI for building Vega-Lite specs**, with
+> **guidance and recommendations on the fly**, **moderately capable** — not a full
+> visual-design IDE.
+>
+> **Decision (2026-06-10):** push the builder from its shipped **Tier B** ("smart +
+> guarded") up to **Tier C** ("intent-first aid"). "Moderately capable" is the ceiling:
+> we add the controls that are _both common and awkward in JSON_ and stop there; the long
+> tail of styling/scale/axis breadth stays in Monaco. The two source docs remain the
+> research record (the _why_ and the citations); this doc is the _plan_ (the _what next_
+> and the _order_).
+
+---
+
+## Status log
+
+Newest first. The at-a-glance build-order tracker is §4; per-item detail is §3. This log is
+the quick "where are we" — read it first.
+
+- **2026-06-10** — **Up next: 1B (per-chart export).**
+  - **1A · Actionable hints** shipped: one-click fixes on guidance warnings
+    (`BuilderWarning.fixes` + `applyWarningFix`), council-reviewed, with focus/announce a11y.
+  - **Builder UX/perf batch** (from dogfooding the Superstore dataset) shipped: near-fullscreen
+    `xlarge` modal tier; panes scroll internally (preview in its own viewport); backdrop-dismiss
+    guard (`dismissOnBackdrop: false`); render-timing diagnostics; **canvas** preview renderer
+    (SVG stays default elsewhere) + **canvas max-dimension guard** (`ChartTooLargeError` via a
+    headless probe); **data-aware default pre-population** (`smartDefaultEncodings`).
+  - Scope consolidated and **Tier-C target** set (this doc created); the research/Lyra forward
+    sequences are superseded by §4 here.
+
+---
+
+## 1. Where the builder is today (the floor — don't rebuild)
+
+Tier B is shipped and tested (M4 done). The intelligence that decides _which chart and
+why_ already exists in `src/core/chart-builder.ts`:
+
+- **Smart default mark** from the (X, Y) field-type shape — not unconditionally Bar
+  (`defaultMark`).
+- **Valid-type-only** field-type menus per column + **Size discipline** (Nominal /
+  Temporal / negative-extent columns are _blocked_ on Size, not merely warned)
+  (`validFieldTypes`, `isChannelTypeAllowed`).
+- **Non-blocking guidance** — `builderWarnings` (6 rules: line/area needs both axes,
+  all-categorical, two-measures-want-scatter, area-split-many-series, crowded category
+  axis, negative-size guard).
+- **Per-channel transforms** — Aggregate / Bin / `timeUnit`, chart-level Sort / Stack,
+  and a field-less "Count of records" measure.
+- **Swap X/Y**, debounced live preview, validation gate (≥1 channel mapped).
+
+Everything in §3 below is **backlog** — verified not yet built: `BuilderWarning` has no
+`fix` field, there is no per-chart export, no top-level `transform`, no data-table
+preview, no channels beyond X/Y/Color/Size, no styling/scale controls.
+
+---
+
+## 2. The target experience (Tier C, concretely)
+
+Reading the brief's four words against the research:
+
+| Brief word                                | What it means here                                                            | The levers (from §3)                                                                                          |
+| ----------------------------------------- | ----------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------- |
+| **Rapid**                                 | shortest path from "a dataset" to "a chart I'd keep"                          | smart defaults (have), intent front door, starter examples, field shelf                                       |
+| **Intuitive**                             | matches how people think ("I have fields; what shows my point?")              | intent front door, field shelf, data preview, value-or-field channels                                         |
+| **Guidance & recommendations on the fly** | the app proposes and corrects, not just validates                             | **actionable hints**, **intent front door** (the defining Tier-C feature)                                     |
+| **Moderately capable**                    | covers the common data-shaping + a few high-value encodings; _not_ every knob | filter / calculate, a _small_ set of promoted controls — and a hard stop short of Lyra's everything-inspector |
+
+The defining shift from B→C is the **intent-first front door**: a _"what do you want to
+show?"_ entry (FT / Datawrapper intent categories) that recommends a mark + channel layout
+from `intent × column-types`, instead of starting the user at a blank mark picker. It is
+the feature the brief most directly asks for, and it is the one piece the M4 research
+_deferred_. Choosing Tier C is choosing to pull it forward.
+
+Tier C **extends** §06 — it does not replace the mark-first builder. The front door is an
+on-ramp; the user can still ignore it and drive the channels directly, and can always drop
+to Monaco. This preserves Astrolabe's core invariant: **the JSON spec is the source of
+truth; the builder is a view that emits it** (the Lyra anti-lesson — never let the GUI
+become the document).
+
+---
+
+## 3. The consolidated enhancement set
+
+Organized into build phases by dependency and value. Each item carries its **source**,
+**value/effort**, **code home**, and **spec impact**. Phases 1–3 are the committed Tier-C
+scope; Phase 4 is explicitly _beyond_ "moderately capable" and gated on a later decision.
+
+### Phase 1 — Guidance + I/O (no new interaction model) — _do first_
+
+High value-to-effort, mostly pure-core + thin UI, no architectural change. These make the
+_current_ builder dramatically better and de-risk the bigger phases.
+
+**1A · Actionable hints** — _done (2026-06-10)_
+Turn advisory warnings into one-click fixes. Source: Lyra §3.1 (`Hints` carries an
+`action`). Several existing warnings have an obvious remedy:
+
+- _"draws one mark per row → "_ **[Aggregate as Sum]** (set the measure's `aggregate`)
+- _"long labels → "_ **[Swap X/Y]** (the action already exists — just wire it)
+- _"two measures usually read as a scatter → "_ **[Switch to Point]**
+- _"area split into many series → "_ **[Stack]** or **[Remove colour]**
+
+Implementation: extended `BuilderWarning` with `fixes?: BuilderWarningFix[]` (`{ label, apply }`, pure,
+unit-tested in `chart-builder.test.ts`); the modal renders each as a ghost button wired to a
+new `applyWarningFix` store action. Wired fixes: **[Aggregate as Sum]** + **[Swap X/Y]**
+(one-mark-per-row), **[Swap X/Y]** (high-cardinality axis), **[Switch to Point]** (two
+measures), **[Stack]** + **[Remove colour]** (area split — and a stacked area is no longer
+flagged, so [Stack] resolves it). Council run (Carbon Actionable notification + APG Alert):
+ghost buttons, remedy-in-button, polite "Applied: …" announcement, focus moved off the
+removed button — resolution recorded in `architecture/10` §5. §06 "Guidance" amended to
+document the one-click fixes.
+
+**1B · Per-chart export** — _highest value-to-effort overall_
+Today export is workspace-backup only (§08); there is **no way to get one chart out**.
+Source: Lyra §3.8. The renderer already holds the live Vega `view`
+(`services/chart-renderer.ts`), so this is small:
+
+- **Copy spec** (clipboard) + **Download `.vl.json`** for the active snippet (trivial — the
+  snippet _is_ the spec)
+- **Download PNG / SVG** via `view.toImageURL('png' | 'svg')`
+- _(optional)_ standalone HTML (ties to the BYO-cloud "private-move" direction)
+
+Home: a **snippet-level** "Export / Share" affordance (library row action or editor
+toolbar), distinct from the workspace Export. _Spec impact: new export surface in §08;
+arguably §02/§03 (where the affordance lives)._ Note: not strictly a _builder_ feature, but
+the biggest single miss adjacent to it — sequence it here.
+
+**1C · Filter (+ Calculate) dataset transforms** — _closes the loop the builder's own warnings open_
+The transform layer the builder doesn't touch: top-level `transform: []`. Source: Lyra
+§3.12. The builder _already tells users to filter_ in three warnings
+(`chart-builder.ts:475,476,511`) while offering no way to do it.
+
+- **Filter** _(highest)_ — a guarded **field + operator + value** predicate shelf
+  (Voyager-style, no expression needed for the common case); power form is a raw `datum.…`
+  expression validated with Vega's `parseExpr` (see 1E). VL applies top-level transforms
+  _before_ encoding aggregation, so "filter raw rows, then aggregate" is the natural
+  default; filtering on an aggregated value (HAVING) is the advanced case — defer.
+- **Calculate / derived field** _(second)_ — `transform: [{calculate, as}]`; the new field
+  then appears in column dropdowns like any other.
+- **Lookup / join a second dataset** — larger data-model change (one dataset per snippet
+  today) → **defer to Phase 4**.
+
+Home: a new **"Data" section in the builder's left pane, above the channels** (`here are
+your rows [+ Filter] [+ Calculate] → now encode them`), paired with 1D. _Spec impact: new
+§06 "Data / transforms" subsection — this is genuinely new behaviour, write it._
+
+**1D · Data-table preview** — _closes a confirmed gap; pairs with 1C_
+Neither the builder nor the Datasets manager ever shows the **actual rows**. Source: Lyra
+§3.2. A compact, **read-only** first-N-rows grid with a **per-column type chip** in each
+header lets users sanity-check inferred types _before_ building — exactly when inference is
+most likely to surprise. Type + cardinality + extent already come from `profile.ts`; we
+only need the row sample. Homes: a collapsible "Data" strip in the builder's left pane
+(under the dataset name) and/or the Datasets manager. Keep it read-only (editing data is
+out of scope). _Spec impact: §06 + §05 (Datasets) additions._
+
+**1E · Inline expression validation + field autocomplete** — _build with 1C, not standalone_
+When 1C's expression mode lands, validate the Vega/VL expression string with the library's
+own `parseExpr` (Lyra §3.6) and surface errors inline; autocomplete the dataset's own
+column names (we have the schema from `profile.ts`) (Lyra §3.10). Record the `parseExpr`
+technique in `architecture/08`. _Spec impact: folded into 1C._
+
+### Phase 2 — Interaction substrate (enables Tier C)
+
+Two changes to _how the user touches fields_. They have standalone value but their main job
+is to be the substrate Phase 3 (and any future added channels) stands on — sequence them
+here so Tier C lands cleanly.
+
+**2A · Value-or-field channels (the Property model)** — _capability gain; medium_
+Source: Lyra §3.3 (`Property.tsx` — one droppable control that is _either_ a literal value
+_or_ a bound field). Today a channel is field-only. Let a channel also hold a **constant
+`value`** (fixed colour / size) with one consistent control and a chip showing the binding
+kind. VL encodes exactly this (`field` vs `value` vs `datum`). Generalizes cleanly to any
+channel we add later. _Spec impact: §06 "Encoding channels" — a channel may carry a
+constant._
+
+**2B · Field shelf + in-place type cycling** — _the largest interaction shift; the Tier-C/facet substrate_
+Source: Lyra §3.4 (drop-zones) + Voyager (field list with type chips) + Lyra §3.5
+(`FieldType` — the type icon _is_ the control, click cycles N→O→Q→T within the valid set).
+Flip from **channel-first** ("pick a channel, then its column") to **field-first** ("here
+are your columns — drag/click onto channels"), matching how people think. This is the
+natural way to assign many fields across many channels, so it is the **interaction
+substrate for Tier C and faceting**, not a standalone task. _Spec impact: §06 layout
+revision (field shelf alongside the channel rows)._
+
+### Phase 3 — Tier C (the intent-first front door) — _the defining feature of this push_
+
+**3A · Intent-first front door** — _the B→C step_
+Source: research §5/§8 Tier C (FT Visual Vocabulary + Datawrapper intent taxonomy). A
+_"what do you want to show?"_ entry mapping **intent × column types → recommended mark +
+channel layout**:
+
+| Intent (FT / Datawrapper)  | Our expression (within 5 marks / our channels)                      |
+| -------------------------- | ------------------------------------------------------------------- |
+| **Magnitude / Comparison** | Bar (x=N, y=Q; horizontal for long labels)                          |
+| **Ranking**                | Bar, sorted by value                                                |
+| **Change over time**       | Line (x=T, y=Q; color=N for series)                                 |
+| **Correlation**            | Point (x=Q, y=Q); Circle + size=Q for a 3rd measure                 |
+| **Distribution**           | Bar of binned counts (histogram) — uses Bin                         |
+| **Part-to-whole**          | Stacked / 100% bar (uses Stack); _true pie needs `theta` → Phase 4_ |
+| **Deviation**              | diverging signed Bar                                                |
+
+Honest coverage gaps stay honest (Spatial / Flow excluded; Part-to-whole partial until
+`theta`). The front door is an **on-ramp, not a gate** — it pre-populates the mark-first
+builder, which the user can then adjust or ignore. Munzner's typology and Wilke's directory
+become seatable council sources at this point (research §2). Built **on the 2B field
+shelf**. **Run the front-door copy + flow through `/council`.** _Spec impact: substantial
+§06 amendment — a new "Intent" front-door subsection; the M4 spec note explicitly flagged
+this as the deferred tier, so this is the planned amendment, not drift._
+
+**3B · Starter examples gallery** — _cheap; pairs with 3A_
+Source: Lyra §3.7. A small set of **curated starter snippets**, one per covered FT intent
+(Magnitude/Bar, Change-over-time/Line, Correlation/Point, Distribution/histogram,
+Part-to-whole/stacked). Improves first-run, doubles as living documentation of what the app
+does well. Natural home: the snippet library. _Spec impact: §02 (library seed content)._
+
+### Builder UX & perf — in-flight fixes (2026-06-10, from dogfooding the Superstore dataset)
+
+Pre-existing builder rough edges surfaced while testing on a 10k-row / ~24-col dataset.
+Fixed in this batch (not part of 1A–3B, but the same surface):
+
+- **Modal is a near-fullscreen work surface** — new `xlarge` shell tier (`ModalShell`,
+  `min(1800px, 96vw) × min(1100px, 92vh)`); the Chart Builder no longer wastes screen.
+- **Panes scroll internally, modal keeps its shape** — the `.builder` grid fills the body
+  (`grid-template-rows: minmax(0,1fr)`), the config pane and the **preview** each scroll in
+  their own viewport, so a tall one-mark-per-row chart scrolls inside the preview instead of
+  pushing Create/Cancel below the fold.
+- **Backdrop click no longer discards in-progress work** — `dismissOnBackdrop: false` on the
+  builder (registry flag); Escape and × still close.
+- **Data-aware default pre-population** — the builder no longer blindly takes the first two
+  columns (which opened Superstore on a 9994-bar degenerate chart). When the dataset is
+  profiled, `defaultBuilderConfig` picks a "safest bet": a low-cardinality category vs a
+  count of records (tidy bar), else a time series of the first measure, else a scatter — each
+  guaranteed to render. Falls back to positional when there are no stats. Pure + tested. This
+  composes with (isn't replaced by) the future intent-first front door — the builder always
+  needs a sane opening state.
+- **Render-timing diagnostics** — `BuilderPreview` logs `parse · prepare · destroy · embed ·
+paint · total` (+ mark, row count) to the console (dev always; prod only when slow). The
+  **paint** phase (a double-rAF after `embed()`) captures the real freeze.
+
+**Perf finding — confirmed and fixed.** Diagnostics on the Superstore dataset:
+`embed 237ms · paint 6458ms` for the default one-bar-per-row chart (9994 rows), vs
+`paint 6ms` once grouped to a few categories. The freeze was entirely **SVG layout/paint**
+(one DOM node per mark), not chart compilation. **Fix shipped:** the builder preview now
+renders with **canvas** (`renderSpec(…, { renderer: 'canvas' })`); SVG stays the default for
+the editor's LivePreview and for image export. Contract divergence recorded in
+`architecture/05` §2.
+
+**Canvas max-dimension guard (measured, not guessed).** Canvas (unlike SVG) has a hard
+max side (~32k px), so a chart that resolves taller than that fails to allocate (the
+broken-image icon). The cause is **physical render size, not cardinality** — a vertical bar
+with thousands of _X_ bands renders fine (width is container-bounded); only an unbounded
+band axis (e.g. a horizontal bar's _Y_) overflows. So `renderSpec` now runs a **headless
+(`'none'`) layout probe** for canvas charts, reads the chart's resolved **height**, and
+throws `ChartTooLargeError(heightPx, limitPx)` when it exceeds `MAX_CANVAS_PX ÷ dpr`. The
+builder catches it and shows the real numbers ("would be ~200,000px tall — larger than the
+browser can draw on a canvas (~16,383px max here); aggregate or filter"). The earlier
+band-count proxy (`previewBandCount`/`MAX_PREVIEW_BANDS`) was removed — readability
+(cardinality) stays a `builderWarnings` concern; the render-size limit is now measured at
+its true cause.
+
+### Phase 4 — Beyond "moderately capable" (gated — decide later)
+
+These exceed the stated ceiling. List them so they have a home, but **do not commit them in
+this push** — revisit once Phases 1–3 land and we see real usage. Each must clear the
+guardrail: _promote a control only when it is **both common AND awkward in JSON**._
+
+- **More channels** — `theta` (unlocks pie/donut → _true_ part-to-whole), `opacity`,
+  `shape`. (`theta` is the most defensible — it closes a real coverage gap.) Ride on 2A/2B.
+- **Faceting (Row / Column → small multiples)** — research §8 B8. The clean way to compare
+  many categories. Design crux: VL facets default to **shared scales** (keep that default);
+  expose an "independent axes" toggle (`resolve.scale`) only as advanced. **Verify against
+  the preview's `"container"` fit modes** (per-cell sizing on facets is finicky).
+- **Light styling / scale controls** — colour-scheme picker (categorical / sequential /
+  diverging), measure-axis `zero` / `log` toggle, custom axis title, legend title / hide.
+  Implement as **auto-derived override panels** that start empty (inheriting VL defaults)
+  and emit JSON **only when touched**; clearing a channel **drops its overrides** (Lyra
+  §3.9 `cleanupUnused` — no orphaned `scale`/`axis` in the output spec).
+- **Builder undo/redo / "reset to smart defaults"** — Lyra §3.11. Low priority (the modal
+  is short-lived; the smart default already gives a sane start).
+- **Lookup / join a second dataset** — Lyra §3.12; data-model change (multi-dataset
+  snippets). Larger, separate effort.
+
+---
+
+## 4. Recommended build order
+
+```
+Phase 1  1A actionable hints      ✓ done
+         1B per-chart export      ← next: highest value-to-effort
+         1C filter (+ calculate)  ← closes the loop on warnings the builder already emits
+         1D data preview          ← pairs with 1C
+         1E expr-validate + autocomplete (with 1C)
+Phase 2  2A value-or-field channels (Property model)
+         2B field shelf + in-place type cycling   ← Tier-C substrate
+Phase 3  3A intent-first front door (Tier C)       ← built on 2B; the defining feature
+         3B starter examples
+Phase 4  (gated) theta/facets/styling-overrides/undo/lookup — decide after Phase 3
+
+Also shipped (builder UX/perf, from dogfooding): near-fullscreen modal, internal-scroll
+panes, backdrop-dismiss guard, render diagnostics, canvas preview + canvas-size guard,
+data-aware default pre-population. See "Builder UX & perf" above.
+```
+
+Rationale for the order: Phase 1 is the cheapest large quality jump and needs no new
+interaction model, so it ships value while the bigger design settles. Phase 2 is pure
+substrate — low _user-visible_ payoff alone, but Phase 3 is much cleaner on top of it than
+bolted onto the channel-first UI. Phase 3 delivers the brief's headline ("recommendations
+on the fly"). Phase 4 is deliberately deferred to protect the "moderately capable" ceiling.
+
+---
+
+## 5. Constraints & anti-scope (the ceiling)
+
+What "moderately capable" rules **out** — load-bearing, from `lyra-review.md` §2.1/§4:
+
+- **No everything-inspector.** Lyra surfaces ~50 direct controls per primitive because the
+  GUI _was_ its document. We split the work on purpose: a small **guarded** builder + a
+  first-class **Monaco** editor for the long tail. Styling/scale/axis breadth for its own
+  sake belongs in Monaco, not the builder.
+- **The promotion test:** a control enters the builder only when it is **both common AND
+  awkward in JSON**. Otherwise it stays in Monaco.
+- **JSON stays the source of truth.** The builder _emits_ spec; it is never the document
+  (the Lyra one-way-export trap that forbids round-trips).
+- **No interaction-by-demonstration, no direct-manipulation canvas, no general
+  data-pipeline editor.** If we ever add interactivity, expose VL `params`/selections as a
+  small guarded action — never port Lyra's signal generator.
+- **Take ideas from the reference clones, read no code into the repo** (`AGENTS.md`: no
+  shared lib; patterns adapted, not imported).
+
+---
+
+## 6. Cross-cutting notes
+
+- **Spec deltas:** 1A (minor §06), 1B (§08 + §02/§03), 1C (new §06 transforms subsection),
+  1D (§06 + §05), 2A/2B (§06 layout), 3A (substantial §06 intent front-door amendment), 3B
+  (§02). Per _"spec follows code now"_: build the decision, then amend the spec to match —
+  don't let code and §06 drift.
+- **Council:** auto-fires on guidance copy and new interactive-widget keyboard/focus work —
+  so 1A (hint affordance + copy) and 3A (front-door flow + copy) both go through `/council`
+  before committing. It advises; architecture 09/10 decide.
+- **Verification:** pure rules get `chart-builder.test.ts` cases; every UI/affordance change
+  gets a manual pass against the _live_ builder — a green build proves nothing about what
+  the user sees (`AGENTS.md`; `docs/manual-verification.md`). Items 1A and 1C are mostly
+  `src/core/`, squarely the "core-first, tested hardest" rule.
+- **Source of truth going forward:** this doc. `chart-builder-research.md` §8 and
+  `lyra-review.md` §5 remain the _research record_; their forward sequences are superseded
+  by §4 here.
diff --git a/docs/chart-builder-research.md b/docs/chart-builder-research.md
index 9e04150..e3b8062 100644
--- a/docs/chart-builder-research.md
+++ b/docs/chart-builder-research.md
@@ -201,8 +201,13 @@ The highest-value guardrails — encodings a naive UI emits that the canon rejec
 ## 8. Future enhancements (backlog)
 
 The Tier-B build is the floor, not the ceiling. The enhancements below were surfaced by
-the research; this is their single home (the milestone plan's M4 row points here). Status
-as of 2026-06-06.
+the research. Status as of 2026-06-06.
+
+> **Forward plan moved (2026-06-10):** the _prioritized, sequenced_ enhancement plan now
+> lives in [`chart-builder-enhancement-scope.md`](./chart-builder-enhancement-scope.md),
+> which merges this backlog with the [`lyra-review.md`](./lyra-review.md) §5 ideas and sets
+> the **Tier-C** target. This section remains the research _record_ (the citations behind
+> each item); consult the scope doc for _what to build next and in what order_.
 
 **A · Cheap wins inside the current 5-mark / 4-channel scope**
 
diff --git a/docs/lyra-review.md b/docs/lyra-review.md
index 70d6d32..8ccbbd7 100644
--- a/docs/lyra-review.md
+++ b/docs/lyra-review.md
@@ -350,6 +350,12 @@ defining features are traps for us.
 
 ## 5. Recommended sequence (mapped to the existing backlog)
 
+> **Consolidated (2026-06-10):** this sequence is now merged with
+> `chart-builder-research.md` §8 into the forward plan at
+> [`chart-builder-enhancement-scope.md`](./chart-builder-enhancement-scope.md) (Tier-C
+> target). The list below is the original Lyra-side reasoning; the scope doc §4 is the
+> authoritative build order.
+
 Slot these into `chart-builder-research.md` §8 rather than inventing a new track:
 
 1. **Actionable hints** (§3.1) — extend `BuilderWarning` with an optional pure `fix`; wire the
diff --git a/docs/spec/06-chart-builder.md b/docs/spec/06-chart-builder.md
index 455c4cb..092427b 100644
--- a/docs/spec/06-chart-builder.md
+++ b/docs/spec/06-chart-builder.md
@@ -56,11 +56,13 @@ These controls appear only when they apply:
 
 ### Default pre-population
 
-- On open, the first detected column is assigned to **X** and the second (if any) to **Y**, each with its derived field type and no transforms. Remaining channels start unmapped. The mark starts at the smart default for that X/Y shape (see _Mark type_), not unconditionally Bar.
+- On open, the builder chooses a **data-aware "safest bet"** so it never opens on a degenerate, unrenderable chart (e.g. a 10k-row dataset whose first two columns are an id and a high-cardinality key would otherwise draw one bar per row). When the dataset is profiled (per-column cardinality available), it prefers, in order: a **low-cardinality category vs a count of records** (a tidy bar); else a **time series** of the first measure over a date; else a **scatter** of two measures. Each is guaranteed to render and read cleanly. The measure for the category case is the field-less **count** deliberately — it is always meaningful and avoids summing an id-like numeric (e.g. a Row ID) into nonsense.
+- When the dataset carries no cardinality stats (older or URL-backed datasets), it falls back to the original positional rule: the first detected column on **X** and the second (if any) on **Y**, each with its derived field type.
+- Either way, remaining channels start unmapped with no transforms, and the mark starts at the smart default for the resulting X/Y shape (see _Mark type_), not unconditionally Bar. The intent-first front door (future) layers richer recommendations on top of this default; it does not replace the need for a sane opening state.
 
 ### Guidance (non-blocking)
 
-The builder surfaces short, plain-language hints for configurations that render but read poorly — advisory only, never blocking the **Create Snippet** action (validation below is the sole gate). These follow the chart-choice research ([`docs/chart-builder-research.md`](../chart-builder-research.md)) and include, for example:
+The builder surfaces short, plain-language hints for configurations that render but read poorly — advisory only, never blocking the **Create Snippet** action (validation below is the sole gate). A hint states the _problem_; where there is an obvious remedy, it also offers one or more **one-click fix** buttons that apply the change to the configuration (e.g. _Aggregate as Sum_, _Swap X/Y_, _Switch to Point_, _Stack_, _Remove colour_). A fix is an offer, never a forced change — applying it updates the config and the hint re-derives away. Interaction/accessibility of these actions follows [`architecture/10`](../architecture/10-interaction-and-feedback.md) §5 (polite announcement, focus moved off the removed button). These follow the chart-choice research ([`docs/chart-builder-research.md`](../chart-builder-research.md)) and include, for example:
 
 - A **Line** or **Area** mark with only one axis mapped (both axes are needed to draw it).
 - A **Bar/Line/Area** whose X and Y are both categories (nothing to measure).
diff --git a/src/app/components/ChartBuilderModal.module.css b/src/app/components/ChartBuilderModal.module.css
index ad7a137..4005832 100644
--- a/src/app/components/ChartBuilderModal.module.css
+++ b/src/app/components/ChartBuilderModal.module.css
@@ -3,7 +3,13 @@
 .builder {
   display: grid;
   grid-template-columns: minmax(320px, 360px) 1fr;
-  min-height: 480px;
+  /* Fill the modal body and never exceed it, so each pane scrolls internally rather
+     than the whole modal growing past the viewport (otherwise a tall chart pushes the
+     Create/Cancel actions below the fold). The `minmax(0, 1fr)` row lets the panes
+     shrink below their content height so their own overflow kicks in. */
+  grid-template-rows: minmax(0, 1fr);
+  height: 100%;
+  min-height: 0;
   min-width: 0;
 }
 
@@ -24,6 +30,7 @@
   border-right: var(--border-width) solid var(--border);
   overflow-y: auto;
   min-width: 0;
+  min-height: 0;
 }
 
 .datasetName {
@@ -213,6 +220,13 @@
   border-radius: var(--radius);
 }
 
+/* These take focus only programmatically (tabIndex -1) after a hint fix is applied,
+   to keep focus off <body>; no visible ring for that script-driven move. */
+.warnings:focus,
+.configPane:focus {
+  outline: none;
+}
+
 .warning {
   display: flex;
   align-items: flex-start;
@@ -230,6 +244,42 @@
   color: var(--support-warning-fg);
 }
 
+/* The hint text and its one-click remedies stacked, growing beside the icon. */
+.warningBody {
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-2);
+  min-width: 0;
+}
+
+/* Actionable-hint remedies: each fix is an offer, never forced, so they're
+   low-emphasis ghost buttons — suggestions beside the advice, not commands. */
+.warningFixes {
+  display: flex;
+  flex-wrap: wrap;
+  gap: var(--space-2);
+}
+
+.warningFix {
+  font: inherit;
+  font-size: 12px;
+  cursor: pointer;
+  padding: var(--space-1) var(--space-2);
+  border: var(--border-width) solid var(--border-strong);
+  border-radius: var(--radius);
+  background: var(--bg);
+  color: var(--accent);
+}
+
+.warningFix:hover {
+  background: var(--layer-01);
+}
+
+.warningFix:focus-visible {
+  outline: 2px solid var(--focus);
+  outline-offset: 1px;
+}
+
 /* Explains the disabled Create action (contract 10: a disabled control must say
    why). `margin-top: auto` pins it just above the actions so the two read as one. */
 .createHint {
@@ -292,6 +342,8 @@
   flex-direction: column;
   padding: var(--space-5);
   min-width: 0;
+  min-height: 0;
+  overflow: hidden;
   background: var(--bg);
 }
 
@@ -305,9 +357,14 @@
 .previewFrame {
   flex: 1;
   min-width: 0;
+  min-height: 0;
   display: flex;
-  align-items: center;
-  justify-content: center;
+  /* A chart taller/wider than the pane scrolls *here*, inside a fixed viewport, so
+     the modal keeps its shape. `safe` centring aligns to the start instead of
+     clipping the top/left when the chart overflows. */
+  overflow: auto;
+  align-items: safe center;
+  justify-content: safe center;
 }
 
 .previewHost {
diff --git a/src/app/components/ChartBuilderModal.test.tsx b/src/app/components/ChartBuilderModal.test.tsx
index 8201e60..1412f3f 100644
--- a/src/app/components/ChartBuilderModal.test.tsx
+++ b/src/app/components/ChartBuilderModal.test.tsx
@@ -8,11 +8,26 @@ import { useSnippetStore } from '../stores/SnippetStore';
 import { ChartBuilderModal } from './ChartBuilderModal';
 
 // The builder preview embeds a real Vega chart in an effect; stub the renderer so
-// this render test stays a pure React/DOM check (the loop we guard against happens
-// during commit, long before any chart is drawn).
-vi.mock('../services/chart-renderer', () => ({
-  renderSpec: () => Promise.resolve({ destroy() {}, resize() {} }),
-}));
+// these tests stay pure React/DOM checks. `renderSpec` is a vi.fn so a test can make
+// it reject (e.g. the canvas-too-large path); the mocked `ChartTooLargeError` is the
+// same class the component imports, so its `instanceof` check matches. The class is
+// declared inside the factory because vi.mock is hoisted above module-scope code.
+vi.mock('../services/chart-renderer', () => {
+  class ChartTooLargeError extends Error {
+    heightPx: number;
+    limitPx: number;
+    constructor(heightPx: number, limitPx: number) {
+      super('too large');
+      this.name = 'ChartTooLargeError';
+      this.heightPx = heightPx;
+      this.limitPx = limitPx;
+    }
+  }
+  return {
+    renderSpec: vi.fn(() => Promise.resolve({ destroy() {}, resize() {} })),
+    ChartTooLargeError,
+  };
+});
 
 // React 19 wants this flag set for act() to drive effects without warnings.
 (globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
@@ -69,6 +84,77 @@ describe('ChartBuilderModal', () => {
     expect(container.textContent).toContain('scatter'); // the guidance hint rendered
   });
 
+  test('a guidance hint offers a one-click fix that resolves it (actionable hints, §06)', async () => {
+    const ds = createDataset({
+      name: 'Nums',
+      data: [
+        { a: 1, b: 2 },
+        { a: 3, b: 4 },
+      ],
+      format: 'json',
+      source: 'inline',
+      now: T,
+    });
+    useDatasetStore.getState().add(ds);
+    const id = useDatasetStore.getState().datasets[0].id;
+    useChartBuilderStore.getState().init(id);
+    useChartBuilderStore.getState().setMark('bar'); // two measures on a bar → scatter hint
+
+    await act(async () => {
+      root.render(<ChartBuilderModal />);
+      await Promise.resolve();
+    });
+
+    // The hint renders a [Switch to Point] button (not just prose).
+    const fixButton = Array.from(container.querySelectorAll('button')).find(
+      (b) => b.textContent === 'Switch to Point',
+    );
+    expect(fixButton).toBeDefined();
+    expect(container.textContent).toContain('scatter');
+
+    await act(async () => {
+      fixButton!.click();
+      await Promise.resolve();
+    });
+
+    // Applying it switches the mark and the hint re-derives away.
+    expect(useChartBuilderStore.getState().config.mark).toBe('point');
+    expect(container.textContent).not.toContain('scatter');
+  });
+
+  test('shows the canvas-limit message when the chart resolves too large to render', async () => {
+    vi.useFakeTimers();
+    const { renderSpec, ChartTooLargeError } = await import('../services/chart-renderer');
+    vi.mocked(renderSpec).mockRejectedValueOnce(new ChartTooLargeError(200_000, 16_383));
+
+    const ds = createDataset({
+      name: 'Big',
+      data: [
+        { a: 1, b: 'x' },
+        { a: 2, b: 'y' },
+      ],
+      format: 'json',
+      source: 'inline',
+      now: T,
+    });
+    useDatasetStore.getState().add(ds);
+    const id = useDatasetStore.getState().datasets[0].id;
+    useChartBuilderStore.getState().init(id);
+
+    await act(async () => {
+      root.render(<ChartBuilderModal />);
+      await Promise.resolve();
+    });
+    // Drive the debounced render so renderSpec runs and rejects with the limit error.
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(400);
+    });
+
+    expect(container.textContent).toContain('larger than the browser can draw on a canvas');
+    expect(container.textContent).toContain('200,000'); // the measured height
+    vi.useRealTimers();
+  });
+
   test('shows the empty state when no dataset is loaded', async () => {
     await act(async () => {
       root.render(<ChartBuilderModal />);
diff --git a/src/app/components/ChartBuilderModal.tsx b/src/app/components/ChartBuilderModal.tsx
index c7a6200..554f4cf 100644
--- a/src/app/components/ChartBuilderModal.tsx
+++ b/src/app/components/ChartBuilderModal.tsx
@@ -33,6 +33,7 @@ import {
   supportsTimeUnit,
   validFieldTypes,
   type AggregateOp,
+  type BuilderWarningFix,
   type ChannelMapping,
   type ChannelName,
   type FieldType,
@@ -42,7 +43,7 @@ import {
 import type { ColumnType } from '@core/type-inference';
 import { DatasetNotFoundError, prepareSpecForRender } from '@core/rendering';
 import { chartConfigFor } from '@core/vega-themes';
-import { renderSpec, type RenderHandle } from '../services/chart-renderer';
+import { ChartTooLargeError, renderSpec, type RenderHandle } from '../services/chart-renderer';
 import { closeModal } from '../modals/ModalCoordinator';
 import { useAppStore } from '../stores/AppStore';
 import { useDatasetStore } from '../stores/DatasetStore';
@@ -58,6 +59,35 @@ import styles from './ChartBuilderModal.module.css';
 
 const RENDER_DEBOUNCE_MS = 300;
 
+/**
+ * Render-timing diagnostics for the builder preview. A many-mark chart (e.g. the
+ * default one-bar-per-row on a 10k-row dataset) is cheap to compile but expensive
+ * for the browser to lay out as **SVG**, and that cost lands *after* `embed()`
+ * resolves, in the next paint — the chart appears, then the tab freezes for a moment.
+ * Each phase is timed, including that post-embed paint (a double rAF lands just after
+ * it), so the numbers attribute the cost to layout rather than chart compilation.
+ * Logged in dev always; in prod only when a render is slow.
+ */
+const SLOW_RENDER_MS = 250;
+function logBuilderRenderTiming(t: {
+  parse: number;
+  prepare: number;
+  destroy: number;
+  embed: number;
+  paint: number;
+  total: number;
+}): void {
+  const total = Math.round(t.total);
+  if (!import.meta.env.DEV && total < SLOW_RENDER_MS) return;
+  const ms = (n: number) => Math.round(n);
+  const { rowCount, config } = useChartBuilderStore.getState();
+  console.info(
+    `[chart-builder] render ${total}ms — parse ${ms(t.parse)} · prepare ${ms(t.prepare)} · ` +
+      `destroy ${ms(t.destroy)} · embed ${ms(t.embed)} · paint ${ms(t.paint)} ` +
+      `(mark=${config.mark}, rows=${rowCount ?? 'n/a'})`,
+  );
+}
+
 /** Title-case a token for display (e.g. `bar` → `Bar`, `sum` → `Sum`). */
 function titleCase(s: string): string {
   return s.charAt(0).toUpperCase() + s.slice(1);
@@ -264,6 +294,9 @@ function BuilderPreview() {
   const handleRef = useRef<RenderHandle | null>(null);
   const generationRef = useRef(0);
   const [error, setError] = useState<string | null>(null);
+  // Set when the chart resolves larger than the canvas backend can draw — a
+  // physical render-size limit, distinct from the readability cardinality warnings.
+  const [tooLarge, setTooLarge] = useState<{ heightPx: number; limitPx: number } | null>(null);
 
   const specText = useChartBuilderStore(selectBuilderSpecText);
   const valid = useChartBuilderStore(selectBuilderValid);
@@ -279,18 +312,26 @@ function BuilderPreview() {
           handleRef.current?.destroy();
           handleRef.current = null;
           setError(null);
+          setTooLarge(null);
           return;
         }
         if (!node) return;
         try {
+          const t0 = performance.now();
           const parsed: unknown = JSON.parse(specText);
+          const t1 = performance.now();
           const prepared = prepareSpecForRender(parsed, { fitMode: 'width', datasets });
-          handleRef.current?.destroy();
+          const t2 = performance.now();
+          handleRef.current?.destroy(); // finalizing a huge prior SVG is itself a cost
           handleRef.current = null;
+          const t3 = performance.now();
           const handle = await renderSpec(
             node,
             prepared as VisualizationSpec,
             chartConfigFor(uiTheme),
+            // Canvas, not SVG: a many-mark preview (one bar per row of a big dataset)
+            // costs seconds of SVG layout/paint; canvas paints in ms (see renderer).
+            { renderer: 'canvas' },
           );
           if (mine !== generationRef.current) {
             handle.destroy();
@@ -298,12 +339,37 @@ function BuilderPreview() {
           }
           handleRef.current = handle;
           setError(null);
+          setTooLarge(null);
+          const t4 = performance.now();
+          // The browser lays out/paints the (possibly huge) SVG after embed resolves;
+          // a double rAF lands just after that paint, capturing the freeze the user
+          // feels. Skipped if a newer render has already superseded this one.
+          requestAnimationFrame(() =>
+            requestAnimationFrame(() => {
+              if (mine !== generationRef.current) return;
+              const t5 = performance.now();
+              logBuilderRenderTiming({
+                parse: t1 - t0,
+                prepare: t2 - t1,
+                destroy: t3 - t2,
+                embed: t4 - t3,
+                paint: t5 - t4,
+                total: t5 - t0,
+              });
+            }),
+          );
         } catch (e) {
           if (mine !== generationRef.current) return;
-          if (e instanceof DatasetNotFoundError) {
+          if (e instanceof ChartTooLargeError) {
+            // A physical render-size limit (canvas max dimension), not a data error.
+            setTooLarge({ heightPx: e.heightPx, limitPx: e.limitPx });
+            setError(null);
+          } else if (e instanceof DatasetNotFoundError) {
             setError(`Dataset "${e.datasetName}" not found.`);
+            setTooLarge(null);
           } else {
             setError(`Couldn't render this chart: ${(e as Error).message}`);
+            setTooLarge(null);
           }
         }
       })();
@@ -325,10 +391,18 @@ function BuilderPreview() {
       {!valid && (
         <p className={styles.previewHint}>Map at least one channel to a column to see a chart.</p>
       )}
-      <div className={styles.previewFrame} hidden={!valid || error !== null}>
+      {valid && tooLarge && (
+        <p className={styles.previewHint} role="status">
+          This chart would be about {Math.round(tooLarge.heightPx).toLocaleString()} px tall —
+          larger than the browser can draw on a canvas (
+          {Math.round(tooLarge.limitPx).toLocaleString()} px max here). Aggregate the measure or
+          filter to fewer rows so it fits.
+        </p>
+      )}
+      <div className={styles.previewFrame} hidden={!valid || tooLarge !== null || error !== null}>
         <div className={styles.previewHost} ref={hostRef} />
       </div>
-      {valid && error !== null && (
+      {valid && tooLarge === null && error !== null && (
         <pre className={styles.previewError} role="alert">
           {error}
         </pre>
@@ -351,6 +425,7 @@ export function ChartBuilderModal() {
   const setStack = useChartBuilderStore((s) => s.setStack);
   const setWidth = useChartBuilderStore((s) => s.setWidth);
   const setHeight = useChartBuilderStore((s) => s.setHeight);
+  const applyWarningFix = useChartBuilderStore((s) => s.applyWarningFix);
   const runCreate = useChartBuilderStore((s) => s.createSnippet);
 
   // Validity + guidance + which chart-level controls apply are derived from the
@@ -369,6 +444,30 @@ export function ChartBuilderModal() {
   const canSort = useMemo(() => supportsSort(config), [config]);
   const canStack = useMemo(() => supportsStack(config), [config]);
 
+  // Applying a hint's fix removes that hint's list item, so focus would otherwise fall
+  // to <body>. The change is announced politely (the chart updates silently for sighted
+  // users) and focus moves to the guidance region, or the config pane if the last hint
+  // just cleared — the pattern for a control that removes its own container (arch 10 §5).
+  const configPaneRef = useRef<HTMLDivElement>(null);
+  const warningsRef = useRef<HTMLUListElement>(null);
+  const pendingFixFocus = useRef(false);
+  const [fixAnnouncement, setFixAnnouncement] = useState('');
+
+  const handleFix = (fix: BuilderWarningFix) => {
+    applyWarningFix(fix); // re-derives `warnings`, firing the focus effect below
+    setFixAnnouncement(`Applied: ${fix.label}.`);
+    pendingFixFocus.current = true;
+  };
+
+  // After a fix re-derives the warnings, move focus off the (now-removed) button:
+  // to the guidance region if hints remain, else the config pane. Ref-flag, not
+  // state, so we never setState inside the effect (react-hooks/set-state-in-effect).
+  useEffect(() => {
+    if (!pendingFixFocus.current) return;
+    pendingFixFocus.current = false;
+    (warningsRef.current ?? configPaneRef.current)?.focus();
+  }, [warnings]);
+
   if (datasetId === null) {
     return <p className={styles.muted}>No dataset loaded. Open this from a dataset in Datasets.</p>;
   }
@@ -381,7 +480,10 @@ export function ChartBuilderModal() {
 
   return (
     <div className={styles.builder}>
-      <div className={styles.configPane}>
+      <div className={styles.configPane} ref={configPaneRef} tabIndex={-1}>
+        <div className="visually-hidden" role="status" aria-live="polite">
+          {fixAnnouncement}
+        </div>
         <p className={styles.datasetName}>
           Building from <strong>{datasetName}</strong>
         </p>
@@ -464,11 +566,32 @@ export function ChartBuilderModal() {
         </div>
 
         {warnings.length > 0 && (
-          <ul className={styles.warnings}>
+          <ul
+            className={styles.warnings}
+            ref={warningsRef}
+            tabIndex={-1}
+            aria-label="Chart guidance"
+          >
             {warnings.map((w) => (
               <li key={w.message} className={styles.warning}>
                 <Icon name="status-warning" className={styles.warningIcon} />
-                <span>{w.message}</span>
+                <div className={styles.warningBody}>
+                  <span>{w.message}</span>
+                  {w.fixes && w.fixes.length > 0 && (
+                    <div className={styles.warningFixes}>
+                      {w.fixes.map((fix) => (
+                        <button
+                          key={fix.label}
+                          type="button"
+                          className={styles.warningFix}
+                          onClick={() => handleFix(fix)}
+                        >
+                          {fix.label}
+                        </button>
+                      ))}
+                    </div>
+                  )}
+                </div>
               </li>
             ))}
           </ul>
diff --git a/src/app/components/ModalShell.module.css b/src/app/components/ModalShell.module.css
index 05963e5..d20cde1 100644
--- a/src/app/components/ModalShell.module.css
+++ b/src/app/components/ModalShell.module.css
@@ -28,6 +28,14 @@
   height: min(700px, 88vh);
 }
 
+/* Extra-large: the Chart Builder — a work surface (config + a chart that wants room),
+   with nothing useful behind it. Near-fullscreen, capped so it doesn't stretch absurdly
+   on ultra-wide displays. Definite height so its panes scroll internally (not the modal). */
+.xlarge {
+  width: min(1800px, 96vw);
+  height: min(1100px, 92vh);
+}
+
 /* Small: single-form modals (Extract). Grows with content up to a cap. */
 .small {
   width: min(560px, 92vw);
diff --git a/src/app/components/ModalShell.tsx b/src/app/components/ModalShell.tsx
index bca4507..8c53280 100644
--- a/src/app/components/ModalShell.tsx
+++ b/src/app/components/ModalShell.tsx
@@ -23,7 +23,11 @@ export function ModalShell() {
   const name = useAppStore((s) => s.activeModal);
   const config = getModalConfig(name);
 
-  const isLarge = name === 'datasets' || name === 'chartBuilder';
+  // The Chart Builder is a near-fullscreen work surface; the Datasets manager is the
+  // standard large two-pane modal; everything else is a small form. Both large kinds
+  // get the static-title initial focus (APG dialog-modal) so content isn't skipped.
+  const isXLarge = name === 'chartBuilder';
+  const isLarge = name === 'datasets' || isXLarge;
 
   // Move focus into the modal on open, return it to the trigger on close. For a
   // large manager (list + detail), APG dialog-modal advises focusing a static
@@ -38,10 +42,16 @@ export function ModalShell() {
   if (!config) return null;
   const Body = config.component;
 
+  // Modals with in-progress work (the Chart Builder's config) opt out of
+  // click-outside-to-close so an accidental backdrop click can't discard it; Escape
+  // and the close button still dismiss. Other modals keep backdrop dismissal.
+  const dismissOnBackdrop = config.dismissOnBackdrop !== false;
+  const sizeClass = isXLarge ? styles.xlarge : isLarge ? styles.large : styles.small;
+
   return (
     <div
       className={styles.backdrop}
-      onClick={() => void closeModal()}
+      onClick={dismissOnBackdrop ? () => void closeModal() : undefined}
       onKeyDown={(e) => {
         if (e.key === 'Escape') {
           e.stopPropagation();
@@ -51,7 +61,7 @@ export function ModalShell() {
     >
       <div
         ref={modalRef}
-        className={`${styles.modal} ${isLarge ? styles.large : styles.small}`}
+        className={`${styles.modal} ${sizeClass}`}
         role="dialog"
         aria-modal="true"
         aria-labelledby="modal-title"
diff --git a/src/app/modals/modal-registry.ts b/src/app/modals/modal-registry.ts
index 2646170..af329c4 100644
--- a/src/app/modals/modal-registry.ts
+++ b/src/app/modals/modal-registry.ts
@@ -42,6 +42,12 @@ export interface ModalConfig {
   getState?: () => Record<string, unknown> | null;
   /** Whether the modal is reflected in the URL hash (navigable). */
   isUrlNavigable?: boolean;
+  /**
+   * Whether a backdrop (click-outside) closes the modal. Defaults to `true`. Set
+   * `false` for modals holding in-progress work an accidental click shouldn't
+   * discard (the Chart Builder) — Escape and the close button still dismiss.
+   */
+  dismissOnBackdrop?: boolean;
 }
 
 export const MODAL_REGISTRY: Partial<Record<ModalName, ModalConfig>> = {
@@ -71,11 +77,14 @@ export const MODAL_REGISTRY: Partial<Record<ModalName, ModalConfig>> = {
   // Opened from a selected dataset's "Build Chart" action; `arg` is its id. Loads
   // the dataset and pre-populates a smart default config (§06). Applies on Create
   // (a new snippet), so there is nothing transient to lose on close — no getState.
+  // Backdrop dismissal is off: the config is real in-progress work, and a stray
+  // click outside this large surface shouldn't throw it away (Escape/× still close).
   chartBuilder: {
     name: 'chartBuilder',
     title: 'Chart Builder',
     component: ChartBuilderModal,
     isUrlNavigable: true,
+    dismissOnBackdrop: false,
     init: (datasetId) => useChartBuilderStore.getState().init(datasetId ? Number(datasetId) : null),
   },
 
diff --git a/src/app/services/chart-renderer.ts b/src/app/services/chart-renderer.ts
index 6737d55..8c61331 100644
--- a/src/app/services/chart-renderer.ts
+++ b/src/app/services/chart-renderer.ts
@@ -27,15 +27,85 @@ export interface RenderHandle {
   resize(): void;
 }
 
-/** Embed a prepared spec into `node`. Non-negotiable: no actions menu, SVG output. */
+export interface RenderOptions {
+  /**
+   * Renderer backend. **Default `'svg'`** — crisp at any zoom, themeable, the
+   * contract default for the editor's LivePreview (docs/architecture/05 §2). The
+   * Chart Builder preview passes **`'canvas'`**: an SVG chart with thousands of
+   * marks (e.g. one bar per row of a 10k-row dataset) costs *seconds* of
+   * main-thread layout/paint per render — measured ~6.5s paint on 9994 rows —
+   * because each mark is a DOM node; canvas is a single node and paints in
+   * milliseconds. Canvas is raster (not crisp on zoom) but that's invisible for an
+   * ephemeral preview, and image export (`view.toImageURL`) is renderer-agnostic.
+   */
+  renderer?: 'svg' | 'canvas';
+}
+
+/**
+ * Maximum canvas side length, in CSS px before the device-pixel-ratio multiplier.
+ * Browsers cap a canvas backing store at ~32767px per side (Chrome/Firefox; Safari
+ * is lower and area-bound); past that the canvas fails to allocate and draws
+ * nothing. SVG has no such cap. `renderSpec` measures a canvas chart's resolved
+ * size against this (÷ dpr, since the backing store is dpr× the CSS size) and
+ * throws `ChartTooLargeError` rather than handing back a blank canvas.
+ */
+export const MAX_CANVAS_PX = 32767;
+
+/**
+ * Thrown by `renderSpec` when a **canvas**-backed chart resolves to a height larger
+ * than the browser can allocate (see `MAX_CANVAS_PX`). Carries the measured size and
+ * the limit so the caller can explain the *actual* cause — the chart is physically
+ * too large to draw — rather than guessing at "too many categories". This is a
+ * render-backend limit, distinct from the readability cardinality warnings.
+ */
+export class ChartTooLargeError extends Error {
+  /** The chart's resolved height, in CSS px. */
+  readonly heightPx: number;
+  /** The per-side limit at the current device-pixel-ratio, in CSS px. */
+  readonly limitPx: number;
+  constructor(heightPx: number, limitPx: number) {
+    super(
+      `Chart is ${Math.round(heightPx)}px tall — over the ~${Math.round(limitPx)}px canvas limit`,
+    );
+    this.name = 'ChartTooLargeError';
+    this.heightPx = heightPx;
+    this.limitPx = limitPx;
+  }
+}
+
+/** The canvas side limit in CSS px at the current display's device-pixel-ratio. */
+function canvasLimitPx(): number {
+  const dpr = typeof window !== 'undefined' ? window.devicePixelRatio || 1 : 1;
+  return MAX_CANVAS_PX / dpr;
+}
+
+/** Embed a prepared spec into `node`. Always: no actions menu. Renderer per `options`. */
 export async function renderSpec(
   node: HTMLElement,
   spec: VisualizationSpec,
   config: Config,
+  options: RenderOptions = {},
 ): Promise<RenderHandle> {
+  const renderer = options.renderer ?? 'svg';
+
+  // Canvas can't allocate past the browser's max dimension, and an oversized canvas
+  // fails *silently* (a blank/broken surface, sometimes a null 2d context). So for
+  // canvas we first run a headless ('none') layout pass — no canvas allocated — read
+  // the chart's resolved height, and throw with the real numbers if it won't fit.
+  // SVG renders any size (just slowly), so it skips this. The probe uses a detached
+  // node and is finalized immediately; only its computed `height` signal is read.
+  if (renderer === 'canvas') {
+    const probeHost = document.createElement('div');
+    const probe = await vegaEmbed(probeHost, spec, { actions: false, renderer: 'none', config });
+    const height = probe.view.height();
+    probe.view.finalize();
+    const limit = canvasLimitPx();
+    if (typeof height === 'number' && height > limit) throw new ChartTooLargeError(height, limit);
+  }
+
   const result: EmbedResult = await vegaEmbed(node, spec, {
     actions: false, // Astrolabe owns its own export/copy affordances
-    renderer: 'svg',
+    renderer,
     config,
   });
 
diff --git a/src/app/stores/ChartBuilderStore.test.ts b/src/app/stores/ChartBuilderStore.test.ts
index 3a8edaf..1e993a6 100644
--- a/src/app/stores/ChartBuilderStore.test.ts
+++ b/src/app/stores/ChartBuilderStore.test.ts
@@ -127,6 +127,19 @@ describe('sort / stack', () => {
   });
 });
 
+describe('applyWarningFix', () => {
+  test('applies a hint fix to the working config (actionable hints, §06)', () => {
+    const id = seedDataset('Nums', [
+      { a: 1, b: 2 },
+      { a: 3, b: 4 },
+    ]);
+    cb().init(id);
+    cb().setMark('bar'); // two quantitative axes on a bar → "scatter" hint with a fix
+    cb().applyWarningFix({ label: 'Switch to Point', apply: (c) => ({ ...c, mark: 'point' }) });
+    expect(cb().config.mark).toBe('point');
+  });
+});
+
 describe('createSnippet', () => {
   test('builds a linked snippet, activates it, and resets the builder', () => {
     const id = seedDataset('Sales', [
diff --git a/src/app/stores/ChartBuilderStore.ts b/src/app/stores/ChartBuilderStore.ts
index 6b078c9c38a61f45e950f49fda37875703d24bf6..2d2f2174d47984860ffa565a3b29666bdd085dba 100644
GIT binary patch
delta 308
zcmZ1x^g3jN35%dpX=YAJYEgJ%QC?<Vx?5(&W^I-NW?zMnj8p}`P#=ZNVukXe%#xDS
zJcYz!g+zsfm{J2ENKi;EPEIUHRVdC^C;=)h&Mz%W22v%7mBk7rsTCzU3VHb@3Q3uH
ziA9x@r?V@uLG*3j#eS8w-m#z{rxIvbdTC}#VjfUmMrK}#da*)&UaD?#PG)kpLQ!gN
zYD%SoMo4CAk*>2sVsc4leqLfy4n(Pr!lC5`W}12mTKZfH3W*@o5FW5n&`8Uyuu{PA
wnWloRokCfDX3FMv-oL`4ShY<SRFD=1YS#nHY82}w=jWwmrfY7lR$R*t02Z2S)Bpeg

delta 33
pcmaDIvLa}M3Cm_bmUQOHY#efo44bt$uCi`!<NGSS`MA;&b^ylY3#I@7

diff --git a/src/core/chart-builder.test.ts b/src/core/chart-builder.test.ts
index 34ed192..6cc9189 100644
--- a/src/core/chart-builder.test.ts
+++ b/src/core/chart-builder.test.ts
@@ -108,16 +108,20 @@ describe('builderWarnings (Tier B advisories)', () => {
     expect(w.some((m) => /need both an X and a Y/.test(m.message))).toBe(true);
   });
 
-  it('warns when two measures are drawn on a non-scatter mark', () => {
-    const w = builderWarnings({
+  it('warns when two measures are drawn on a non-scatter mark, offering [Switch to Point]', () => {
+    const config: BuilderConfig = {
       datasetName: 'D',
       mark: 'bar',
       encodings: {
         x: { field: 'a', type: 'quantitative' },
         y: { field: 'b', type: 'quantitative' },
       },
-    });
-    expect(w.some((m) => /scatter/.test(m.message))).toBe(true);
+    };
+    const w = builderWarnings(config);
+    const hint = w.find((m) => /scatter/.test(m.message));
+    const fix = hint?.fixes?.find((f) => f.label === 'Switch to Point');
+    expect(fix).toBeDefined();
+    expect(fix!.apply(config).mark).toBe('point');
   });
 
   it('warns when a bar/line/area has no measure on either axis', () => {
@@ -129,8 +133,8 @@ describe('builderWarnings (Tier B advisories)', () => {
     expect(w.some((m) => /need a measure/.test(m.message))).toBe(true);
   });
 
-  it('warns when an area chart is split into colour series', () => {
-    const w = builderWarnings({
+  it('warns when an area chart is split into colour series, offering [Stack] / [Remove colour]', () => {
+    const config: BuilderConfig = {
       datasetName: 'D',
       mark: 'area',
       encodings: {
@@ -138,8 +142,20 @@ describe('builderWarnings (Tier B advisories)', () => {
         y: { field: 'v', type: 'quantitative' },
         color: { field: 'g', type: 'nominal' },
       },
-    });
-    expect(w.some((m) => m.channel === 'color')).toBe(true);
+    };
+    const w = builderWarnings(config);
+    const hint = w.find((m) => m.channel === 'color');
+    expect(hint).toBeDefined();
+    const labels = hint?.fixes?.map((f) => f.label) ?? [];
+    expect(labels).toEqual(['Stack', 'Remove colour']); // most-recommended first
+    // [Remove colour] clears the colour channel, so the hint re-derives away.
+    const cleared = hint!.fixes!.find((f) => f.label === 'Remove colour')!.apply(config);
+    expect(cleared.encodings.color).toBeNull();
+    expect(builderWarnings(cleared).some((m) => m.channel === 'color')).toBe(false);
+    // [Stack] turns it into a part-to-whole stack, which is no longer flagged.
+    const stacked = hint!.fixes!.find((f) => f.label === 'Stack')!.apply(config);
+    expect(stacked.stack).toBe('zero');
+    expect(builderWarnings(stacked).some((m) => m.channel === 'color')).toBe(false);
   });
 
   it('is silent for a clean configuration', () => {
@@ -170,7 +186,29 @@ describe('builderWarnings (Tier B advisories)', () => {
       const hint = w.find((m) => /one mark per row/.test(m.message));
       expect(hint?.channel).toBe('x'); // the category axis
       expect(hint?.message).toContain('406 in this dataset');
-      expect(hint?.message).toMatch(/Swap X\/Y/); // bar → horizontal-bar remedy
+      // Remedies are one-click fixes, not prose; most-recommended first.
+      const labels = hint?.fixes?.map((f) => f.label) ?? [];
+      expect(labels).toEqual(['Aggregate as Sum', 'Swap X/Y']); // aggregate before swap
+    });
+
+    it('[Aggregate as Sum] resolves the one-mark-per-row hint', () => {
+      const w = crowded();
+      const hint = w.find((m) => /one mark per row/.test(m.message));
+      const fix = hint?.fixes?.find((f) => f.label === 'Aggregate as Sum');
+      expect(fix).toBeDefined();
+      const fixed = fix!.apply({
+        datasetName: 'D',
+        mark: 'bar',
+        encodings: {
+          x: { field: 'name', type: 'nominal' },
+          y: { field: 'mpg', type: 'quantitative' },
+        },
+      });
+      expect(fixed.encodings.y?.aggregate).toBe('sum');
+      // and the hint is gone once applied
+      expect(builderWarnings(fixed, 406).some((m) => /one mark per row/.test(m.message))).toBe(
+        false,
+      );
     });
 
     it('is silent once the measure is aggregated (one bar per category)', () => {
@@ -219,8 +257,9 @@ describe('builderWarnings (Tier B advisories)', () => {
       );
       const hint = w.find((m) => /one mark per row/.test(m.message));
       expect(hint).toBeDefined();
-      expect(hint?.message).not.toMatch(/Swap X\/Y/);
-      expect(hint?.message).toMatch(/reduce the number of categories/);
+      const labels = hint?.fixes?.map((f) => f.label) ?? [];
+      expect(labels).not.toContain('Swap X/Y'); // a horizontal line makes no sense
+      expect(labels).toContain('Aggregate as Sum'); // aggregate still applies
     });
   });
 
@@ -262,7 +301,7 @@ describe('builderWarnings (Tier B advisories)', () => {
       const hint = w.find((m) => /distinct values/.test(m.message));
       expect(hint?.channel).toBe('x');
       expect(hint?.message).toMatch(/48 distinct values/);
-      expect(hint?.message).toMatch(/Swap X\/Y/); // bar → horizontal-bar remedy
+      expect(hint?.fixes?.map((f) => f.label)).toContain('Swap X/Y'); // horizontal-bar remedy
     });
 
     it('reports "more than 50" when the category cardinality hit the profiler cap', () => {
@@ -426,7 +465,8 @@ describe('builderWarnings (Tier B advisories)', () => {
 });
 
 describe('defaultBuilderConfig', () => {
-  it('puts the first column on X and the second on Y, each with derived type', () => {
+  it('falls back to first-on-X, second-on-Y when the dataset is unprofiled', () => {
+    // `columns` carries no columnStats → no data-aware pick → positional default.
     const config = defaultBuilderConfig('Sales', columns);
     expect(config.mark).toBe('bar');
     expect(config.datasetName).toBe('Sales');
@@ -467,6 +507,91 @@ describe('defaultBuilderConfig', () => {
   });
 });
 
+describe('defaultBuilderConfig — data-aware "safest bet" (profiled datasets)', () => {
+  /** Stats for one column. */
+  const stat = (name: string, distinct: number, capped = false) => ({
+    name,
+    distinct,
+    distinctCapped: capped,
+    numericExtent: null,
+  });
+
+  it('opens on a low-cardinality category vs a count of records, not the first two columns', () => {
+    // Superstore-shaped: an id-like number first, a high-cardinality id, then tidy
+    // categories — the case a positional first-two-columns default would open as a
+    // 9994-bar degenerate chart.
+    const wide: BuilderColumns = {
+      columns: ['Row ID', 'Order ID', 'Segment', 'Sales'],
+      columnTypes: [
+        { name: 'Row ID', type: 'number' },
+        { name: 'Order ID', type: 'string' },
+        { name: 'Segment', type: 'string' },
+        { name: 'Sales', type: 'number' },
+      ],
+      columnStats: [
+        stat('Row ID', 50, true),
+        stat('Order ID', 50, true), // high cardinality → not a category axis
+        stat('Segment', 3), // tidy category → the pick
+        stat('Sales', 50, true),
+      ],
+    };
+    const config = defaultBuilderConfig('Superstore', wide);
+    expect(config.mark).toBe('bar');
+    expect(config.encodings.x).toEqual({ field: 'Segment', type: 'nominal' });
+    expect(config.encodings.y).toEqual({ type: 'quantitative', aggregate: 'count' });
+  });
+
+  it('picks the lowest-cardinality readable category among several', () => {
+    const cols: BuilderColumns = {
+      columns: ['Region', 'Segment', 'City'],
+      columnTypes: [
+        { name: 'Region', type: 'string' },
+        { name: 'Segment', type: 'string' },
+        { name: 'City', type: 'string' },
+      ],
+      columnStats: [stat('Region', 4), stat('Segment', 3), stat('City', 50, true)],
+    };
+    const config = defaultBuilderConfig('D', cols);
+    expect(config.encodings.x).toEqual({ field: 'Segment', type: 'nominal' }); // 3 < 4
+  });
+
+  it('falls through to a time series (date vs count) when no tidy category exists', () => {
+    const cols: BuilderColumns = {
+      columns: ['Order ID', 'Order Date', 'Sales'],
+      columnTypes: [
+        { name: 'Order ID', type: 'string' },
+        { name: 'Order Date', type: 'date' },
+        { name: 'Sales', type: 'number' },
+      ],
+      columnStats: [
+        stat('Order ID', 50, true),
+        stat('Order Date', 50, true),
+        stat('Sales', 50, true),
+      ],
+    };
+    const config = defaultBuilderConfig('D', cols);
+    expect(config.mark).toBe('line');
+    expect(config.encodings.x).toEqual({ field: 'Order Date', type: 'temporal' });
+    expect(config.encodings.y).toEqual({ field: 'Sales', type: 'quantitative' }); // the measure, raw
+  });
+
+  it('falls through to a scatter of two measures when there is no category or date', () => {
+    const cols: BuilderColumns = {
+      columns: ['Order ID', 'Sales', 'Profit'],
+      columnTypes: [
+        { name: 'Order ID', type: 'string' },
+        { name: 'Sales', type: 'number' },
+        { name: 'Profit', type: 'number' },
+      ],
+      columnStats: [stat('Order ID', 50, true), stat('Sales', 50, true), stat('Profit', 50, true)],
+    };
+    const config = defaultBuilderConfig('D', cols);
+    expect(config.mark).toBe('point');
+    expect(config.encodings.x).toEqual({ field: 'Sales', type: 'quantitative' });
+    expect(config.encodings.y).toEqual({ field: 'Profit', type: 'quantitative' });
+  });
+});
+
 describe('isBuilderConfigValid', () => {
   const base: BuilderConfig = { datasetName: 'D', mark: 'bar', encodings: {} };
 
diff --git a/src/core/chart-builder.ts b/src/core/chart-builder.ts
index b2fa571..9b34b92 100644
--- a/src/core/chart-builder.ts
+++ b/src/core/chart-builder.ts
@@ -230,14 +230,87 @@ function fieldTypeForColumn(name: string, columns: BuilderColumns): FieldType {
   return defaultFieldType(match?.type ?? 'string');
 }
 
+/**
+ * Above this many distinct values, a column is too high-cardinality to be a good
+ * default category axis: its labels overlap into an unreadable axis, and an
+ * unaggregated chart that wide can exceed the canvas size limit. Matches the
+ * crowded-axis warning threshold.
+ */
+const CATEGORY_DEFAULT_MAX_DISTINCT = 30;
+
+/**
+ * Pick a *sensible, renderable* default X/Y from the data shape, using profiled
+ * cardinality (`columnStats`). This avoids opening the builder on a degenerate
+ * one-mark-per-row chart — the shape a positional first-two-columns rule yields when
+ * the leading columns are an id and a high-cardinality key. Returns `null` when there
+ * are no stats to reason about (older / URL datasets), so the caller falls back to the
+ * positional default.
+ *
+ * Preference order (each guaranteed to render and read cleanly):
+ *   1. a low-cardinality **category** vs a **count of records** → a tidy bar;
+ *   2. else a **temporal** axis vs count → a time series (continuous x, always fits);
+ *   3. else two **measures** → a scatter (continuous axes, always fit).
+ *
+ * The category case pairs with the field-less **count**, not a raw measure: count is
+ * always meaningful and avoids summing an id-like numeric (Row ID, Postal Code) into
+ * nonsense. This is the builder's *opening* state only; a later intent-first entry
+ * point can layer richer recommendations on top.
+ */
+function smartDefaultEncodings(
+  columns: BuilderColumns,
+): { x: ChannelMapping; y: ChannelMapping } | null {
+  const stats = columns.columnStats;
+  if (!stats || stats.length === 0) return null; // no profiling → positional fallback
+  const typeOf = (name: string): ColumnType =>
+    columns.columnTypes.find((c) => c.name === name)?.type ?? 'string';
+  const knownDistinct = (name: string): number | undefined => {
+    const s = stats.find((x) => x.name === name);
+    return s && !s.distinctCapped ? s.distinct : undefined;
+  };
+  const count: ChannelMapping = { type: 'quantitative', aggregate: 'count' };
+  const numbers = columns.columns.filter((name) => typeOf(name) === 'number');
+
+  // 1. The lowest-cardinality readable category (string/boolean) → a tidy bar. Pair it
+  //    with **count** (not a raw measure): a raw measure would draw one bar per row.
+  const category = columns.columns
+    .filter((name) => {
+      const t = typeOf(name);
+      if (t !== 'string' && t !== 'boolean') return false;
+      const d = knownDistinct(name);
+      return d !== undefined && d >= 2 && d <= CATEGORY_DEFAULT_MAX_DISTINCT;
+    })
+    .sort((a, b) => (knownDistinct(a) ?? 0) - (knownDistinct(b) ?? 0))[0];
+  if (category) return { x: { field: category, type: 'nominal' }, y: count };
+
+  // 2. A date → a time series of the first measure (a temporal axis is continuous, so a
+  //    line of raw values always fits); fall back to count if there is no measure.
+  const temporal = columns.columns.find((name) => typeOf(name) === 'date');
+  if (temporal) {
+    const y: ChannelMapping = numbers[0] ? { field: numbers[0], type: 'quantitative' } : count;
+    return { x: { field: temporal, type: 'temporal' }, y };
+  }
+
+  // 3. Two measures → a scatter (continuous axes, always renderable).
+  if (numbers.length >= 2) {
+    return {
+      x: { field: numbers[0], type: 'quantitative' },
+      y: { field: numbers[1], type: 'quantitative' },
+    };
+  }
+  return null;
+}
+
 /**
  * The builder's opening configuration for a dataset (spec §06 → Default
- * pre-population, Tier B): the first column on X and the second (if any) on Y, each
- * with its derived field type; Color and Size start unmapped, no transforms. The
- * mark is the **smart default** for the resulting X/Y shape (`defaultMark`) rather
- * than always Bar — a date-vs-number dataset opens as a Line, two measures as a
- * Point — so the first preview is already the conventional chart. A dataset with no
- * detected columns yields an all-unmapped config (the modal then prompts).
+ * pre-population, Tier B). When the dataset is profiled, X/Y are chosen as a
+ * **data-aware "safest bet"** (`smartDefaultEncodings`) — a low-cardinality category
+ * vs a count of records (a tidy bar), else a time series, else a scatter — so the
+ * builder never opens on a degenerate one-mark-per-row chart that can't render. When
+ * there are no stats to reason about, it falls back to the positional rule: first
+ * column on X, second (if any) on Y, each with its derived field type. Either way the
+ * mark is the **smart default** for the resulting X/Y shape (`defaultMark`), Color
+ * and Size start unmapped with no transforms, and a dataset with no detected columns
+ * yields an all-unmapped config (the modal then prompts).
  */
 export function defaultBuilderConfig(datasetName: string, columns: BuilderColumns): BuilderConfig {
   const encodings: Partial<Record<ChannelName, ChannelMapping | null>> = {
@@ -246,12 +319,20 @@ export function defaultBuilderConfig(datasetName: string, columns: BuilderColumn
     color: null,
     size: null,
   };
-  const [first, second] = columns.columns;
-  if (first !== undefined) {
-    encodings.x = { field: first, type: fieldTypeForColumn(first, columns) };
-  }
-  if (second !== undefined) {
-    encodings.y = { field: second, type: fieldTypeForColumn(second, columns) };
+  // Prefer a data-aware "safest bet" (a renderable, readable chart) when the dataset
+  // is profiled; otherwise fall back to the positional first-on-X, second-on-Y rule.
+  const smart = smartDefaultEncodings(columns);
+  if (smart) {
+    encodings.x = smart.x;
+    encodings.y = smart.y;
+  } else {
+    const [first, second] = columns.columns;
+    if (first !== undefined) {
+      encodings.x = { field: first, type: fieldTypeForColumn(first, columns) };
+    }
+    if (second !== undefined) {
+      encodings.y = { field: second, type: fieldTypeForColumn(second, columns) };
+    }
   }
   const mark = defaultMark(encodings.x?.type ?? null, encodings.y?.type ?? null);
   return { datasetName, mark, encodings };
@@ -329,12 +410,27 @@ export function supportsStack(config: BuilderConfig): boolean {
   );
 }
 
+/**
+ * A one-click remedy a warning can offer. `apply` is a pure config→config transform;
+ * the modal renders `label` as a button that runs it. It is an *offer*, never a forced
+ * change — once applied, the warning re-derives away. Lives in core so the remedies
+ * unit-test alongside the warnings.
+ */
+export interface BuilderWarningFix {
+  /** The button label naming the remedy, e.g. "Aggregate as Sum". */
+  label: string;
+  /** Produce the corrected configuration from the current one (pure). */
+  apply: (config: BuilderConfig) => BuilderConfig;
+}
+
 /** A non-blocking advisory about a configuration (spec §06 → Tier B warnings). */
 export interface BuilderWarning {
   /** The channel the hint is about, when it's channel-specific. */
   channel?: ChannelName;
   /** A short, plain-language hint the modal shows inline (not an error). */
   message: string;
+  /** Optional one-click remedies the modal renders as buttons next to the hint. */
+  fixes?: BuilderWarningFix[];
 }
 
 /**
@@ -371,6 +467,41 @@ function cardinalityText(stats: ColumnStats): string {
   return stats.distinctCapped ? `more than ${DISTINCT_CAP}` : `${stats.distinct}`;
 }
 
+// --- Pure config transforms backing the actionable-hint fixes. They mirror the
+// store's setChannelAggregate / swapXY / setStack / setChannelColumn(null) / setMark
+// actions, so applying a fix and making the equivalent manual edit land on the same
+// config. Kept here (not the store) so the remedies are pure and unit-testable. ---
+
+/** Aggregate one channel's field (clearing any bin — the two are mutually exclusive). */
+function withChannelAggregate(
+  config: BuilderConfig,
+  channel: ChannelName,
+  aggregate: AggregateOp,
+): BuilderConfig {
+  const current = config.encodings[channel];
+  if (!current) return config;
+  const next: ChannelMapping = { ...current, aggregate };
+  delete next.bin;
+  return { ...config, encodings: { ...config.encodings, [channel]: next } };
+}
+
+/** Exchange the X and Y mappings (the manual Swap X/Y, as a pure transform). */
+function withSwappedXY(config: BuilderConfig): BuilderConfig {
+  return {
+    ...config,
+    encodings: {
+      ...config.encodings,
+      x: config.encodings.y ?? null,
+      y: config.encodings.x ?? null,
+    },
+  };
+}
+
+/** Clear one channel back to "None" (drops its mapping from the spec). */
+function withChannelCleared(config: BuilderConfig, channel: ChannelName): BuilderConfig {
+  return { ...config, encodings: { ...config.encodings, [channel]: null } };
+}
+
 /**
  * Non-blocking advisories for the current configuration (spec §06 → Tier B): the
  * encodings that render but read poorly, drawn from the research's soft rules
@@ -427,17 +558,25 @@ export function builderWarnings(
     mark !== 'circle'
   ) {
     warnings.push({
-      message: 'Two measures usually read best as a scatter — try Point or Circle.',
+      message: 'Two measures usually read best as a scatter.',
+      fixes: [{ label: 'Switch to Point', apply: (c) => ({ ...c, mark: 'point' }) }],
     });
   }
 
   // Area split into many series hides per-component change (FT Visual Vocabulary:
   // "seeing change in components can be very difficult").
-  if (mark === 'area' && config.encodings.color) {
+  // (Stacking turns overlapping series into a cumulative part-to-whole, a valid read,
+  // so a stacked area is not flagged — applying the [Stack] fix below clears this.)
+  if (mark === 'area' && config.encodings.color && !config.stack) {
+    const fixes: BuilderWarningFix[] = [];
+    if (supportsStack(config)) {
+      fixes.push({ label: 'Stack', apply: (c) => ({ ...c, stack: 'zero' }) });
+    }
+    fixes.push({ label: 'Remove colour', apply: (c) => withChannelCleared(c, 'color') });
     warnings.push({
       channel: 'color',
-      message:
-        'Area charts make per-series change hard to read; consider Line for multiple series.',
+      message: 'Area charts make per-series change hard to read.',
+      fixes,
     });
   }
 
@@ -455,28 +594,38 @@ export function builderWarnings(
   // long category lists belong on a horizontal bar).
   if (mark === 'bar' || mark === 'line' || mark === 'area') {
     const category = sortableCategoryChannel(config); // discrete axis of a category-vs-measure pair
-    const measure = category ? config.encodings[category === 'x' ? 'y' : 'x'] : null;
-    if (category && measure) {
+    const measureChannel = category ? (category === 'x' ? 'y' : 'x') : null;
+    const measure = measureChannel ? (config.encodings[measureChannel] ?? null) : null;
+    if (category && measureChannel && measure) {
       const rawMeasure = !measure.aggregate && !measure.bin;
       if (rawMeasure && typeof rowCount === 'number' && rowCount > CROWDED_CATEGORY_ROWS) {
-        const fix =
-          mark === 'bar'
-            ? 'Aggregate the measure (e.g. Sum or Mean) for one bar per category, or use Swap X/Y for a horizontal bar where long labels stay readable.'
-            : 'Aggregate the measure (e.g. Sum or Mean) so there is one mark per category, or reduce the number of categories.';
+        // Aggregating the measure collapses one-mark-per-row to one-per-category; a bar
+        // can also flip horizontal (Swap X/Y) where long labels stay readable. Offer
+        // aggregate only for a quantitative measure (Sum is meaningless on a date).
+        const fixes: BuilderWarningFix[] = [];
+        if (effectiveType(measure) === 'quantitative') {
+          fixes.push({
+            label: 'Aggregate as Sum',
+            apply: (c) => withChannelAggregate(c, measureChannel, 'sum'),
+          });
+        }
+        if (mark === 'bar') fixes.push({ label: 'Swap X/Y', apply: withSwappedXY });
         warnings.push({
           channel: category,
-          message: `This draws one mark per row (${rowCount} in this dataset), so the category-axis labels will overlap. ${fix}`,
+          message: `This draws one mark per row (${rowCount} in this dataset), so the category-axis labels will overlap.`,
+          fixes: fixes.length ? fixes : undefined,
         });
       } else {
         const stats = statsFor(config.encodings[category]?.field, columns);
         if (stats && stats.distinct > CROWDED_CATEGORY_DISTINCT) {
-          const fix =
-            mark === 'bar'
-              ? 'Use Swap X/Y for a horizontal bar where long lists stay readable, or filter to fewer categories.'
-              : 'Filter to fewer categories, or group the long tail into an "Other".';
+          // Aggregated already, so the remedy is fewer categories (filter — not yet a
+          // builder control) or, for a bar, a horizontal flip where long lists fit.
+          const fixes: BuilderWarningFix[] =
+            mark === 'bar' ? [{ label: 'Swap X/Y', apply: withSwappedXY }] : [];
           warnings.push({
             channel: category,
-            message: `This category axis has ${cardinalityText(stats)} distinct values, so its labels will overlap. ${fix}`,
+            message: `This category axis has ${cardinalityText(stats)} distinct values, so its labels will overlap.`,
+            fixes: fixes.length ? fixes : undefined,
           });
         }
       }