# 06 · Chart Builder

The Chart Builder is a visual, no-JSON way to compose a Vega-Lite chart from a selected dataset. The user picks a mark type and maps the dataset's columns to encoding channels; the builder produces a complete Vega-Lite spec and saves it as a new snippet that references the dataset. It is intended for users who want to start a chart quickly without hand-writing JSON in the _Spec Editor & Draft/Published Workflow_.

> **Design level — "smart + guarded" (Tier B).** The builder is field-first — the user works from a shelf of the dataset's columns and drops them onto encoding channels — and stays within the inputs below, but it is not a dumb composer: it picks a sensible default mark for the data shape, offers only field types valid for each column, keeps unsuitable channel mappings out of reach, and surfaces non-blocking guidance for encodings that render poorly. These behaviors are derived from cross-source chart-choice research recorded in [`docs/exploration/chart-builder-research.md`](../exploration/chart-builder-research.md) (the convergence of Draco, Voyager, the FT Visual Vocabulary, and Datawrapper). On top of this "smart + guarded" base sits the **intent-first front door** (Tier C — _what do you want to show?_, see _Intent_ below): an on-ramp that recommends a whole chart from the user's stated intent, without replacing the mark-first builder beneath it.

## Opening

The builder has several doors; the guided creation path must be visible where the intent to make a chart forms, not buried behind the data model.

- **The library's creation surface** — the primary **Build Chart** action (see _Snippet Library → The List_). Opens the builder with no preselected dataset; the builder picks the **most recently modified dataset** itself.
- **The onboarding canvas** — the data-first "Build a chart from your data" door (see _Snippet Library → First-Run & Empty Workspace_).
- **A dataset's "Build Chart" action** in the _Datasets_ manager — the contextual shortcut; opens the builder on that dataset.
- Opens as a modal dialog over the application; the URL reflects the loaded dataset's "build" form (or `#build` while no dataset is loaded) so the open builder is shareable/restorable (see _Application Shell & Navigation_).
- On open, the builder loads its dataset and pre-populates sensible defaults (see below).

### Dataset picker

Which data the chart builds from is itself a builder choice. A **Dataset** picker sits at the top of the configuration pane, showing the loaded dataset and letting the user switch to any other without leaving the builder.

- Switching while the configuration is still the untouched opening default **re-derives fresh smart defaults** for the new dataset.
- Switching after the user has built on the configuration **rebases** it instead: chart-level intent survives (mark, title/subtitle, explicit size, sort/stack, calculated fields, expression filters), while bindings to columns the new dataset lacks are shed (encodings cleared, predicate filters dropped). A same-schema dataset — the common switch — keeps everything.

### No datasets

With an empty dataset library the builder shows a **no-datasets state** instead of controls: it says what the builder does and offers one primary next step — **Add a dataset**, which opens the _Datasets_ manager on its create form. The guided path never dead-ends.

### Intent (the front door)

A persistent **"What do you want to show?"** strip sits at the top of the configuration pane, under the dataset picker — the builder's guided on-ramp (Tier C). It offers a small set of analytic **intents**, each of which, when chosen, **sets the whole chart up for you** ("do it for me", modelled on Tableau's _Show Me_):

- The intents map the FT Visual Vocabulary / Datawrapper taxonomy onto the builder's marks and channels: **Compare** (magnitude across categories → a bar of counts), **Ranking** (the same, sorted by value), **Change over time** (a line of the first measure over a date), **Correlation** (a scatter of two measures), **Distribution** (a histogram — a binned measure vs count), **Part-to-whole** (a stacked bar split by a second category), and **Heatmap** (a two-category grid shaded by count).
- Picking an intent reshapes the chart — its **mark, encodings, and sort/stack** — to that intent's recommended layout, derived from the dataset's column roles. It **keeps** the dataset, the data transforms (filters / calculated fields), and the chart properties (title/subtitle/size): those are orthogonal to _what kind of chart_.
- The strip is an **on-ramp, not a gate**: it seeds the mark-first builder, which the user can then adjust freely or ignore entirely (and can always drop to Monaco). The chosen intent is builder-local steering — it **never enters the produced spec** (the JSON stays the document).
- On open, the strip **pre-highlights the intent matching the data-aware default** (so a category-vs-count default opens on _Compare_). The highlight is **derived from the configuration, not stored**: the chip whose recommended layout the live chart currently matches stays highlighted; once the user edits away from any recommended layout, none is highlighted — a **Custom** chart.
- Intents the dataset **cannot satisfy** are **disabled** (Tableau _Show Me_): a _Correlation_ needs two number columns, a _Heatmap_ or _Part-to-whole_ needs two category columns, _Change over time_ needs a date, and so on. A disabled chip stays perceivable and carries the reason in its accessible name; it is never hidden.
- Keyboard/focus follow [`architecture/10`](../architecture/10-interaction-and-feedback.md) §5 (an APG **toolbar**: one tab stop, a roving tabindex, arrow keys move focus, Enter/Space applies the intent — so navigation never reshapes the chart by accident).

## Layout

A two-pane modal:

- **Left — configuration:** the dataset picker, a **Data** section (row filters, calculated fields, and a collapsible row preview — see _Data_ below), mark type selector, a **field shelf** (the dataset's columns, field-first), a **Marks** card (the Colour and Size encodings), and a "Create Snippet" action. Chart-level properties (title/subtitle, width/height) sit on the **preview side**, under the chart (see _Chart properties_).
- **Right — live preview:** the **Columns** (X) and **Rows** (Y) shelves stacked above a rendered chart that updates as the configuration changes, with a placeholder/error area. Position is a property of the chart, so its controls sit on the chart (Tableau's Columns/Rows metaphor).

## Data (preview, filters, calculated fields)

A **Data** section sits at the top of the configuration pane — "here are your rows; shape them, then encode them". It previews the source rows first, then offers controls to shape them before encoding. Everything here is optional; a chart can be built with none of it. The shaping controls emit the spec's top-level `transform` array (see _Output_).

The section is ordered **input → shaping** so the distinction reads at a glance: the row preview (the **input** data) comes first, the filters and calculated fields (which shape what the chart actually draws) come below it.

### Data preview

- The first item in the section: a collapsible, **read-only** sample of the dataset's first rows (capped), with a per-column **type chip** in each header. It lets the user sanity-check inferred types against the actual values _before_ building — exactly when type inference is most likely to surprise. Editing the data is out of scope. A non-tabular payload (a single JSON object, TopoJSON, an unfetched URL) has no rows to show.
- The preview shows the dataset's **raw source rows** — _before_ the filters and calculated fields below are applied; it does not show derived columns. Its position above those controls makes that explicit: it is the input, not the result. The transformed result is shown by the **data inspector** under the chart on the preview side (below), which switches between input and resolved rows read from the rendered view.

### Filters

- A list of row filters, each added via **Add filter** and removable. All filters combine (logical **AND**) and apply to the raw rows **before** any encoding aggregation, so "filter rows, then aggregate" is the natural reading.
- A filter is, by default, a **guarded predicate** — a **field**, an **operator**, and a **value** — needing no expression for the common case:
  - The **field** dropdown offers the dataset's columns plus any calculated fields.
  - The **operators** offered depend on the field's type: a measure or temporal field offers `is` / `is not` / `<` / `≤` / `>` / `≥` / `is between` (two bounds); a category offers `is` / `is not` / `is one of` (a comma-separated membership list). A quantitative value compares as a number; other types compare as text (ISO dates sort correctly as text).
- A filter can be switched to an **expression** power-mode — a raw Vega predicate expression (e.g. `datum.value > 0`) — for what the guarded shelf can't say. The toggle is reversible.
- An **incomplete** filter (no value yet, a blank range bound, an empty or syntactically-invalid expression) is ignored, so the live preview keeps rendering while the user types.

### Calculated fields

- A list of derived fields, each added via **Add field** and removable: a **name** and a **Vega expression** that produces a new column (e.g. `profit` = `datum.revenue - datum.cost`).
- A named calculated field appears in the encoding **channel dropdowns** like any real column (it defaults to **Quantitative**, the common arithmetic case, and its type can be overridden on the channel within the valid set). Calculated fields are applied **before** filters, so a filter may reference a derived field.
- Removing or renaming a calculated field that a channel referenced **clears that channel** (the produced spec never encodes a field that no longer exists).

### Expression validation

- Both expression inputs (a filter's expression mode, a calculated field) are validated with **Vega's own expression parser** — the same one the chart uses — so a syntax error is reported **inline** the moment it appears, matching exactly what the chart would accept.
- A `datum.<field>` reference that does not match a known column raises a soft **"unknown field"** warning (a typo guard) without blocking — the value is genuinely valid Vega, it just won't resolve.
- An expression that does not parse is **left out of the produced spec** (like an incomplete filter or an unnamed calculated field): the inline error is the only feedback, and a half-typed expression never reaches the renderer — the preview keeps showing the last valid chart.

## Inputs and Controls

### Mark type

- Single selection from an exact set of six mark types: **Bar, Line, Point, Area, Circle, Heatmap**. **Heatmap** is the Vega-Lite `rect` mark — an X×Y grid of cells shaded by a Colour measure; it is labelled by the chart it makes rather than its geometry, since "Rect" is opaque to the no-JSON audience.
- On open, the mark **defaults to the type that best fits the pre-populated X/Y field-type shape** (Tier B smart default): a temporal axis against a measure → **Line**; two measures → **Point**; a category against a measure → **Bar**; two categories → **Point**; and **Bar** as the fallback when only one axis (or none) is mapped. **Heatmap is never the auto-default** — it reads only with a Colour measure, which the X/Y shape alone can't determine, so it stays a deliberate pick (guidance nudges the missing Colour). The user can switch to any of the six afterward.
- Exactly one mark type is active at any time; selecting one updates the preview.

### Encoding channels

Exactly four channels are offered: **X, Y** (the positional axes, on the on-chart Columns/Rows shelves) and **Colour, Size** (the Marks card). Assignment is **field-first**.

- **The field shelf** lists the dataset's detected columns (see _Datasets_), each with a small type glyph, plus any calculated fields and a field-less **"Count of records"** measure (Vega-Lite `count`). Past a threshold of columns the shelf groups into **Dimensions** (categories/dates) and **Measures** (numerics); a small dataset stays a single flat list. A column already mapped somewhere is dimmed (it may still be placed on more than one channel).
- **Assigning a field:** clicking a shelf field opens an explicit **channel chooser** listing the channels that accept it (an occupied channel is labelled with what it would replace); picking one places the field there. With a channel **armed**, the chooser is skipped — the clicked field fills the armed channel directly. A channel slot is armed by clicking it; arming is visible at the shelf (an accent ring plus a status line naming the target — "Assigning to X — choose a field below. Esc cancels"), and **Esc** disarms without closing the builder. A field that no channel can take offers no choices.
- **A mapped channel is a pill:** a leading type chip, the field (or "Count") label, and a remove (×). The type chip **is the field-type control** — activating it opens a **direct pick** of the types **valid for that column** and channel (Tier B valid-type locking): number → {Quantitative (default), Ordinal, Nominal}; date → {Temporal}; text → {Nominal (default), Ordinal}; boolean → {Nominal}. When only one type applies (e.g. a date), the chip is inert. A fresh mapping defaults its type from the inferred column type (numeric → Quantitative, date → Temporal, otherwise Nominal).
- **Constant values (the Property model):** the **Colour** and **Size** channels may instead hold a **fixed constant** — a literal colour or size applied to every mark, emitted as Vega-Lite `{ value }` rather than a field binding. An empty Colour/Size slot offers a **"Use a constant"** ghost button; the bound constant shows a colour picker (Colour) or a number (Size). X and Y stay field-only (a constant position is not useful). Switching a channel between a field and a constant is reversible, and the prior field type is preserved across the toggle.
- **Size discipline:** the **Size** channel accepts only columns whose natural type is a magnitude (numeric) — size implies an ordered magnitude, so categories and dates are not placed on Size by assignment (they remain available on X/Y/Colour). A constant size is always allowed.
- **Clearing** a channel (the pill's ×) leaves it out of the produced spec.
- A **Swap X/Y** control, by the on-chart shelves, exchanges the X and Y bindings (field/constant and type) in one click — for quickly flipping the axes of the pre-populated default.

### Faceting placeholder (reserved)

Each on-chart shelf shows, beside its axis slot, a **non-interactive placeholder** for **faceting → small multiples** (a future capability). It only signals where row/column faceting will live; it does nothing yet.

### Transforms (per channel)

Once a column is mapped, the channel offers the transforms that apply to its field type — and only those:

- **Aggregate** (any field — the menu narrows by field type): a Quantitative field offers `Sum`, `Mean`, `Median`, `Min`, `Max`, `Count distinct`, or `None`; a Temporal or Ordinal field offers `Min`, `Max`, `Count distinct` (an ordering but no arithmetic); a Nominal field offers `Count distinct` alone. `Count distinct` counts a field's unique values, so the channel reads as a **quantitative measure** whatever the field's own type (e.g. "unique customers per region" on a Color or Y channel); the field's asserted type is preserved and restored when the aggregate is removed. (The field-less `Count` measure is chosen via the "Count of records" column option above.)
- **Bin** (a Quantitative field): bins the values into ranges — e.g. a Quantitative X binned with a Count Y is a histogram. Binning and aggregating the same field are mutually exclusive (setting one clears the other).
- **Granularity** (a Temporal field): a Vega-Lite `timeUnit` — Year, Year-Quarter, Year-Month, Year-Month-Day, Quarter, Month, Week, Day of month, Day of week, Hour — or `None` (raw timestamps). Defaults to **None** (no silent change to what the raw data shows).

### Sort and stacking (chart-level)

These controls appear only when they apply:

- **Sort** (when X and Y form a category-vs-measure pair): sorts the categorical axis by the measure — `Ascending`, `Descending`, or `None` — the standard way to rank a bar chart.
- **Stacking** (a Bar or Area mark with a Color series): `Stacked` (absolute) or `100%` (normalized, part-to-whole). Bars/areas without a Color series, or other marks, show no stacking control.

### Default pre-population

- 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). 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/exploration/chart-builder-research.md`](../exploration/chart-builder-research.md)) and include, for example:

- A **Line**, **Area**, or **Heatmap** 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).
- A **Heatmap** with both axes mapped but **no measure on Colour** (its cells have nothing to shade) — offers a one-click _Colour by count_, the canonical cross-tab heatmap. _Two measures on a heatmap are exempt from the scatter nudge below: a binned 2-D histogram is two quantitative axes shaded by count._
- **Two measures** on a non-scatter mark (a scatter — Point/Circle — usually reads better; Heatmap excepted).
- An **Area** chart split into multiple colour series (per-series change is hard to see).
- A **Bar/Line/Area** that pairs a category axis with a **raw (un-aggregated) measure** over a many-row dataset — it draws one mark, and one axis label, per row, so the category axis becomes an unreadable picket fence. The hint suggests aggregating the measure (one mark per category) or, for a bar, flipping to a horizontal bar (Swap X/Y) where long labels stay readable (FT Visual Vocabulary / Datawrapper). Only the un-aggregated case (mark-count = row-count) is detected; flagging an _aggregated_ axis that still has many distinct categories needs per-column distinct counts the profiler does not yet compute (a known gap).

A clean configuration shows no hints.

### Chart properties (optional)

A slim strip pinned **under the live preview** — these describe the chart itself, so they live on the chart side rather than in the configuration pane:

- **Title** and **Subtitle** text inputs, written into the spec's top-level `title` (a bare string for a lone title; the `{ text, subtitle }` object form when both are set). A subtitle is emitted only alongside a title — Vega-Lite has no standalone subtitle — so the Subtitle input is disabled until a title exists. A non-empty title is also preferred verbatim as the created snippet's name.
- Optional numeric **Width** and **Height** inputs in pixels.
- When left empty, sizing is default/responsive (consistent with _Live Preview_); when provided, the values are written into the spec **and the builder preview renders at that explicit size** (the preview's fit-to-pane sizing applies only while sizing is auto).

## Live Preview

- The right pane renders the chart described by the current mark, encodings, and dimensions, resolving the dataset reference to its actual data (same rendering behavior as _Live Preview_).
- Updates are debounced: changes to mark, encodings, or dimensions trigger a re-render after a short pause rather than on every keystroke.
- While no encoding is mapped, the pane shows a placeholder instructing the user to configure at least one encoding.
- If the spec fails to render, the pane shows an inline error message describing the problem instead of a chart.
- Under the chart, the same **data inspector** as _Live Preview_ (Input | Resolved rows, collapsed by default) lets the user compare the source rows against what the builder's filters and calculated fields produced. It appears only with a live chart, so it never doubles the placeholder/error.

## Validation

- A chart requires **at least one** channel bound — a field, a count, or a constant value.
- While nothing is bound, the "Create Snippet" action is disabled and the preview shows the configuration prompt.

## Output / Create

Selecting "Create Snippet" produces the final artifact:

- Builds a complete Vega-Lite spec containing: the schema reference, a named data reference to the dataset, any top-level `transform` (calculated fields first, then row filters — see _Data_), the chosen mark (with tooltips enabled), the bound encodings (a field encoding carries its field and field type plus any aggregate / bin / `timeUnit` transform; a constant encoding is a `{ value }`), chart-level sort and stacking where set, any title/subtitle, and any explicit width/height.
- Channels left unmapped are omitted; if no encodings exist the spec omits the encoding block entirely (prevented by validation here).
- Creates a new snippet from that spec with an auto-generated descriptive name, adds it to the snippet library, and records that it was built from the dataset.
- Links the snippet to the dataset by recording the dataset reference, so the bidirectional snippet↔dataset relationship is established (see _Datasets_).
- Closes the builder; the newly created snippet becomes the active snippet in the library/editor. **No success toast** — the result is immediately visible (the new snippet opens in the editor), so a toast would be noise (architecture 10 §1, "toast only what the user can't already see"). This refines the earlier blanket "every action toasts" rule, consistent with the Extract-to-dataset / publish reconciliation.

## Open in builder (edit in place)

The builder is also the way to **revise** a chart it could have produced — not only create one. The **editor toolbar** (top of the Spec Editor, alongside the Draft/Published toggle, Config, Revert, and Publish) offers an **Open in builder** action that reopens the active snippet in the builder, populated from its spec — the visual counterpart to editing the same chart's JSON, placed where that editing happens.

- **When it is offered.** Only when the active snippet's **published spec** is **losslessly representable** in the builder's dialect _and_ the dataset it references still exists. Losslessness is judged by parsing the spec back to a builder configuration and **re-assembling it for an exact comparison** against the original (ignoring key order, the `$schema` stamp, and the builder's injected `tooltip`), not by enumerating supported features — so the gate stays correct automatically as the builder's dialect grows. A spec the builder cannot reproduce exactly (hand-authored richness, an unsupported channel/mark, inline `data.values`, a `url` source) stays **Monaco-only**, and the action is **hidden** for it — the same content-gated treatment as _Extract to Dataset_ (a permanently-disabled control the user can't enable in the moment would read as broken; see _Interaction & Feedback_ → action visibility). The builder references a dataset **by name** — its sole data model — so only a snippet that references a saved dataset can hydrate.
- **What opening does.** Hydrates the builder from the snippet — mark, encodings, transforms, sort/stack, title/subtitle, size — and loads the referenced dataset's columns. The configuration pane names the snippet under edit ("Editing _name_"), and the dataset picker behaves as a **rebase** (built-on work) for any subsequent dataset switch — a loaded chart is never treated as a fresh default.
- **An edit session is builder-local, not part of the URL.** Like the builder's in-progress configuration generally, the "editing _name_" context is transient: a reload, Back, or shared link reopens the builder as a fresh **create** flow on the same dataset rather than restoring the edit. No data is lost — the published snippet is untouched until _Save changes_ — and re-entering the edit is one click from the toolbar.
- **Saving.** The primary action becomes **Save changes** (in place of "Create Snippet"): it **republishes** the built spec into the same snippet — overwriting **both** its published and draft versions, so there is no pending draft to reconcile — while keeping the snippet's identity, timestamps' `created`, and dataset links. A user-chosen name is preserved; an auto-named snippet re-derives its name from the new content (as _Publish_ does). The edited snippet becomes the active snippet. As with Create, there is no success toast (the result is immediately visible) and the action is gated on the same validation (at least one channel bound).
- **The JSON stays the document.** Because Open-in-builder is strict, the builder never silently overwrites a richer spec it cannot represent — it is a view that emits the spec, never a competing source of truth.

## Closing

- The builder can be dismissed without creating anything (close control / modal dismissal).
- Closing resets all builder state (dataset, mark type, encodings, dimensions, preview) so a later open starts fresh, and any pending preview render is cancelled.
