Chart builder: SelectControl pickers, channel chooser, per-type aggregates

docs/spec/06-chart-builder.md

@@ -14,7 +14,7 @@ The Chart Builder is a visual, no-JSON way to compose a Vega-Lite chart from a s
 
 A two-pane modal:
 
-- **Left — configuration:** dataset name, 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), optional width/height inputs, and a "Create Snippet" action.
+- **Left — configuration:** dataset name, 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)
@@ -62,9 +62,9 @@ The section is ordered **input → shaping** so the distinction reads at a glanc
 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 places it on the **armed** channel, else the first empty channel that accepts it (X→Y→Colour→Size). A channel slot is armed by clicking it (then a clicked field fills it); the armed slot shows a "pick a field" prompt. A field that no channel can take is a no-op.
-- **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 cycles the field's type within the set **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 an "or constant" affordance; 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.
+- **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.
@@ -77,7 +77,7 @@ Each on-chart shelf shows, beside its axis slot, a **non-interactive placeholder
 
 Once a column is mapped, the channel offers the transforms that apply to its field type — and only those:
 
-- **Aggregate** (a measure / Quantitative field): one of `Sum`, `Mean`, `Median`, `Min`, `Max`, or `None`. (The field-less `Count` measure is chosen via the "Count of records" column option above.)
+- **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).
 
@@ -106,10 +106,13 @@ The builder surfaces short, plain-language hints for configurations that render
 
 A clean configuration shows no hints.
 
-### Dimensions (optional)
+### 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, the chart uses default/responsive sizing (consistent with _Live Preview_); when provided, the values are written into the spec.
+- 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
 
@@ -127,7 +130,7 @@ A clean configuration shows no hints.
 
 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, and any explicit width/height.
+- 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_).