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

src/app/components/ChartBuilderModal.module.css

@@ -103,18 +103,6 @@
   gap: var(--space-2);
 }
 
-.filterField {
-  flex: 1;
-  min-width: 0;
-  padding: var(--space-2) var(--space-3);
-  border: var(--border-width) solid var(--border-strong);
-  border-radius: var(--radius);
-  background: var(--bg);
-  color: var(--text);
-  font: inherit;
-  font-size: 13px;
-}
-
 .filterPredicate {
   display: flex;
   align-items: center;
@@ -173,7 +161,6 @@
   font-family: var(--font-mono);
 }
 
-.filterField:focus-visible,
 .calcName:focus-visible,
 .valueInput:focus-visible,
 .exprInput:focus-visible {
@@ -428,16 +415,6 @@
   color: var(--text-placeholder);
 }
 
-.mini {
-  padding: var(--space-1) var(--space-2);
-  border: var(--border-width) solid var(--border-strong);
-  border-radius: var(--radius);
-  background: var(--bg);
-  color: var(--text);
-  font: inherit;
-  font-size: 12px;
-}
-
 .toggle {
   display: inline-flex;
   align-items: center;
@@ -451,31 +428,11 @@
   gap: var(--space-5);
 }
 
-.mini:focus-visible,
 .dimInput:focus-visible {
   outline: 2px solid var(--focus);
   outline-offset: -1px;
 }
 
-.dimensions {
-  display: flex;
-  flex-direction: column;
-  gap: var(--space-2);
-}
-
-.dimInputs {
-  display: flex;
-  gap: var(--space-3);
-}
-
-.dimField {
-  display: flex;
-  flex-direction: column;
-  gap: var(--space-1);
-  font-size: 12px;
-  color: var(--text-secondary);
-}
-
 .dimInput {
   width: 100px;
   padding: var(--space-2) var(--space-3);
@@ -713,6 +670,44 @@
   color: var(--text-secondary);
 }
 
+/* Scroll viewport around the field lists: a wide dataset (Superstore: 25 columns)
+   must not push the Marks card and chart controls below the fold. Group headings
+   pin to the top while their list scrolls under them. */
+.shelfScroll {
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-2);
+  max-height: 280px;
+  overflow-y: auto;
+}
+
+/* An armed channel makes the shelf the live target — say so where the next click
+   happens (NN/g #1): an accent ring on the list plus the status line above it. */
+.shelfArmed {
+  outline: 2px solid var(--accent);
+  outline-offset: 2px;
+  border-radius: var(--radius);
+}
+
+.armedHint {
+  margin: 0;
+  font-size: 12px;
+  color: var(--text-secondary);
+}
+
+.armedHint strong {
+  color: var(--accent);
+}
+
+.shelfScroll .shelfGroupHead {
+  position: sticky;
+  top: 0;
+  z-index: 1;
+  margin-top: 0;
+  padding: var(--space-2) 0;
+  background: var(--layer-01);
+}
+
 .shelfList {
   display: flex;
   flex-direction: column;
@@ -785,11 +780,14 @@
   gap: var(--space-2);
 }
 
-/* Pill — a bound channel (field/count or constant). */
+/* Pill — a bound channel (field/count or constant). The per-channel transform
+   controls (Aggregate/Bin/Granularity) flow inline beside the pill, wrapping only
+   when the row truly runs out of room (a narrow window; the Marks card). */
 .pillWrap {
   display: flex;
-  flex-direction: column;
-  gap: var(--space-2);
+  flex-wrap: wrap;
+  align-items: center;
+  gap: var(--space-2) var(--space-3);
   min-width: 0;
 }
 
@@ -827,6 +825,10 @@
   opacity: 0.85;
 }
 
+.pillType[aria-expanded='true'] {
+  background: color-mix(in srgb, #000 30%, var(--accent));
+}
+
 .pillType:focus-visible {
   outline: 2px solid var(--accent-contrast);
   outline-offset: -2px;
@@ -841,6 +843,8 @@
 }
 
 .pillName {
+  /* Grow into any pill slack so the remove ✕ stays at the pill's right edge. */
+  flex: 1;
   min-width: 0;
   padding: var(--space-2) var(--space-3);
   overflow: hidden;
@@ -937,19 +941,22 @@
   outline-offset: 1px;
 }
 
+/* Ghost button (Carbon: links navigate, buttons act — this one acts). */
 .slotConst {
-  padding: var(--space-1);
+  padding: var(--space-1) var(--space-2);
   border: none;
+  border-radius: var(--radius);
   background: none;
   color: var(--accent);
   font: inherit;
   font-size: 11px;
   white-space: nowrap;
-  text-decoration: underline;
   cursor: pointer;
+  transition: background var(--dur-fast) var(--ease);
 }
 
 .slotConst:hover {
+  background: var(--layer-02);
   color: var(--accent-hover);
 }
 
@@ -966,10 +973,12 @@
   padding: var(--space-5) var(--space-5) 0;
 }
 
+/* Swap sits beside the heading — an action flushed to the far edge of a very wide
+   pane reads as unrelated to the shelves it acts on. */
 .shelvesHead {
   display: flex;
   align-items: baseline;
-  justify-content: space-between;
+  gap: var(--space-4);
 }
 
 .shelfStrip {
@@ -1000,11 +1009,53 @@
   padding: var(--space-2) var(--space-3);
 }
 
-.shelfSlots .slot,
-.shelfSlots .pillWrap {
+/* Empty slots keep a minimum presence; a filled pill hugs its content so the
+   remove ✕ sits at its right edge rather than adrift in dead pill background. */
+.shelfSlots .slot {
   min-width: 190px;
 }
 
+/* Chart properties — title/subtitle/size as a slim strip pinned under the preview
+   (chart-level properties live on the chart side; council 2026-06-12). */
+.chartProps {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: var(--space-4);
+  padding: var(--space-3) var(--space-5);
+  border-top: var(--border-width) solid var(--border);
+}
+
+.propField {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--space-2);
+  font-size: 12px;
+  color: var(--text-secondary);
+  white-space: nowrap;
+}
+
+.propInput {
+  width: 200px;
+  padding: var(--space-1) var(--space-2);
+  border: var(--border-width) solid var(--border-strong);
+  border-radius: var(--radius);
+  background: var(--bg);
+  color: var(--text);
+  font: inherit;
+  font-size: 12px;
+}
+
+.propInput:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}
+
+.propInput:focus-visible {
+  outline: 2px solid var(--focus);
+  outline-offset: -1px;
+}
+
 /* Reserved faceting placeholder (Phase 4) — non-interactive, signals where it'll live. */
 .facetSlot {
   display: flex;

src/app/components/ChartBuilderModal.test.tsx

@@ -194,7 +194,7 @@ describe('ChartBuilderModal', () => {
       await vi.advanceTimersByTimeAsync(400); // drive the debounced preview render
     });
 
-    expect(container.querySelector('select[aria-label="Filter operator"]')).toBeTruthy();
+    expect(container.querySelector('button[aria-label^="Filter operator"]')).toBeTruthy();
     const calls = vi.mocked(renderSpec).mock.calls;
     const lastSpec = calls[calls.length - 1][1] as { transform?: unknown };
     expect(lastSpec.transform).toEqual([{ filter: { field: 'revenue', gt: 60 } }]);
@@ -297,7 +297,7 @@ describe('ChartBuilderModal', () => {
     expect(refLink()!.getAttribute('href')).toContain('vega.github.io');
   });
 
-  test('clicking a field in the shelf assigns it to a channel as a pill (field-first, 2B)', async () => {
+  test('clicking a field opens the channel chooser; picking a channel assigns it (field-first, 2B)', async () => {
     const ds = createDataset({
       name: 'Shop',
       data: [{ region: 'E', sales: 5 }],
@@ -309,7 +309,6 @@ describe('ChartBuilderModal', () => {
     const id = useDatasetStore.getState().datasets[0].id;
     const store = useChartBuilderStore.getState();
     store.init(id);
-    // Clear the smart-default axes so the click lands on the first empty channel (X).
     store.setChannelColumn('x', null);
     store.setChannelColumn('y', null);
 
@@ -328,12 +327,100 @@ describe('ChartBuilderModal', () => {
       await Promise.resolve();
     });
 
+    // Unarmed, the click opens an explicit channel chooser (portaled to <body>)
+    // rather than silently filling the first empty seat (council 2026-06-12).
+    expect(useChartBuilderStore.getState().config.encodings.x).toBeNull();
+    const option = Array.from(document.body.querySelectorAll('button')).find((b) =>
+      b.textContent?.includes('Columns (X)'),
+    );
+    expect(option).toBeDefined();
+
+    await act(async () => {
+      option!.click();
+      await Promise.resolve();
+    });
+
     expect(useChartBuilderStore.getState().config.encodings.x).toEqual({
       field: 'region',
       type: 'nominal',
     });
   });
 
+  test('an armed channel short-circuits the chooser: the field assigns directly (2B)', async () => {
+    const ds = createDataset({
+      name: 'Shop',
+      data: [{ region: 'E', sales: 5 }],
+      format: 'json',
+      source: 'inline',
+      now: T,
+    });
+    useDatasetStore.getState().add(ds);
+    const id = useDatasetStore.getState().datasets[0].id;
+    const store = useChartBuilderStore.getState();
+    store.init(id);
+    store.setChannelColumn('x', null);
+    store.setChannelColumn('y', null);
+    store.focusChannel('y');
+
+    await act(async () => {
+      root.render(<ChartBuilderModal />);
+      await Promise.resolve();
+    });
+
+    // The armed state announces itself at the shelf.
+    expect(container.textContent).toContain('Assigning to Y');
+
+    const fieldButton = Array.from(container.querySelectorAll('button')).find((b) =>
+      b.textContent?.includes('region'),
+    );
+    await act(async () => {
+      fieldButton!.click();
+      await Promise.resolve();
+    });
+
+    expect(useChartBuilderStore.getState().config.encodings.y).toEqual({
+      field: 'region',
+      type: 'nominal',
+    });
+    expect(useChartBuilderStore.getState().activeChannel).toBeNull();
+  });
+
+  test('opening a SelectControl lands focus on the selected option, not the first (regression)', async () => {
+    const ds = createDataset({
+      name: 'Sales',
+      data: [{ day: '2026-01-01', v: 1 }],
+      format: 'json',
+      source: 'inline',
+      now: T,
+    });
+    useDatasetStore.getState().add(ds);
+    const id = useDatasetStore.getState().datasets[0].id;
+    const store = useChartBuilderStore.getState();
+    store.init(id);
+    store.setChannelColumn('x', 'day'); // temporal → the pill offers Granularity
+    store.setChannelTimeUnit('x', 'month'); // "Month" sits mid-list, after "None (raw)"
+
+    await act(async () => {
+      root.render(<ChartBuilderModal />);
+      await Promise.resolve();
+    });
+
+    const trigger = container.querySelector<HTMLButtonElement>(
+      'button[aria-label^="Granularity for"]',
+    );
+    expect(trigger).toBeTruthy();
+    await act(async () => {
+      trigger!.click();
+      await Promise.resolve();
+    });
+
+    // A selector list ('[aria-current="true"], button') would return the first
+    // button in document order — the "None (raw)" option — instead of the selection.
+    const focused = document.activeElement as HTMLElement;
+    expect(focused.getAttribute('aria-current')).toBe('true');
+    expect(focused.textContent).toContain('Month');
+  });
+
   test('Colour can be switched to a constant value (the Property model, 2A/2B)', async () => {
     const ds = createDataset({
       name: 'Shop',
@@ -351,9 +438,10 @@ describe('ChartBuilderModal', () => {
       await Promise.resolve();
     });
 
-    // The empty Colour slot offers an "or constant" affordance (Colour is first in Marks).
+    // The empty Colour slot offers a "Use a constant" ghost button (Colour is
+    // first in Marks).
     const constButton = Array.from(container.querySelectorAll('button')).find(
-      (b) => b.textContent === 'or constant',
+      (b) => b.textContent === 'Use a constant',
     );
     expect(constButton).toBeDefined();
 

src/app/components/ChartBuilderModal.tsx

@@ -1,15 +1,19 @@
 /**
  * Chart Builder — the modal body (spec §06).
  *
- * A two-pane composer: left is the configuration (dataset name, mark selector, one
- * block per channel, chart-level sort/stacking, optional dimensions, guidance,
- * Create), right is a live preview of the spec the configuration produces. All spec
- * logic and Tier-B defaults/guards come from `@core/chart-builder` via
- * `ChartBuilderStore`; this component is the view. Each channel is a small block:
- * a column dropdown (with a field-less "Count of records" option), a fixed
- * `N | O | Q | T` field-type segmented control (the column's invalid types are
- * disabled), and the transforms that apply to its type (aggregate + bin for a
- * measure, granularity for a temporal field). The preview is builder-local (its own
+ * A two-pane composer. Left: the Data section (filters, calculated fields, row
+ * preview), the mark selector, the field shelf (columns as type-glyphed chips), the
+ * Marks card (Colour/Size — field or constant), chart-level sort/stacking, guidance,
+ * and Create. Right: the on-chart Columns/Rows shelves, the live preview, and the
+ * chart-properties strip (title/subtitle/size). All spec logic and defaults/guards
+ * come from `@core/chart-builder` via `ChartBuilderStore`; this component is the view.
+ *
+ * Assignment is field-first: a chip click opens an explicit channel chooser, or
+ * assigns directly when a channel is armed (the slot's "Pick a field…" state, made
+ * visible at the shelf). A bound channel renders as a pill — a field-type chip that
+ * opens a direct type pick, the field name, a remove ✕ — with its per-type
+ * transforms (aggregate / bin / granularity) inline beside it. Pickers use
+ * `SelectControl`, not native selects. The preview is builder-local (its own
  * debounced render over the shared `chart-renderer` service) rather than a reuse of
  * `LivePreview`, which is bound to the snippet editor's stores.
  */
@@ -29,12 +33,14 @@ import {
   filterOpArity,
   isBuilderConfigValid,
   isChannelTypeAllowed,
+  isColumnAllowedOnChannel,
   isValueMapping,
   supportsAggregate,
   supportsBin,
   supportsSort,
   supportsStack,
   supportsTimeUnit,
+  validAggregateOps,
   validFieldTypes,
   validFilterOps,
   type BuilderCalculate,
@@ -65,6 +71,7 @@ import {
   useChartBuilderStore,
 } from '../stores/ChartBuilderStore';
 import { SegmentedControl, type SegmentedOption } from './SegmentedControl';
+import { SelectControl } from './SelectControl';
 import { Icon } from './Icon';
 import styles from './ChartBuilderModal.module.css';
 
@@ -124,8 +131,16 @@ const TYPE_ABBR: Record<FieldType, string> = {
   temporal: 'T',
 };
 
-/** Non-count aggregate operators offered for a quantitative field. */
-const FIELD_AGGREGATES: readonly AggregateOp[] = ['sum', 'mean', 'median', 'min', 'max'];
+/** Readable labels for the non-count aggregate operators (`validAggregateOps`
+ *  supplies the per-type menu — e.g. a Nominal field offers only Count distinct). */
+const AGGREGATE_LABELS: Record<Exclude<AggregateOp, 'count'>, string> = {
+  sum: 'Sum',
+  mean: 'Mean',
+  median: 'Median',
+  min: 'Min',
+  max: 'Max',
+  distinct: 'Count distinct',
+};
 
 /** Friendly labels for each temporal granularity. */
 const TIME_UNIT_LABELS: Record<TimeUnit, string> = {
@@ -286,14 +301,9 @@ function ChannelPill({
 
   const count = isCount(mapping);
   const typeOptions = count ? [] : channelTypeOptions(channel, mapping, columns);
-  const canCycle = typeOptions.length > 1;
+  const canPick = typeOptions.length > 1;
   const currentType: FieldType = count ? 'quantitative' : mapping.type;
   const label = count ? 'Count' : (mapping.field ?? '');
-  const cycleType = () => {
-    if (!canCycle) return;
-    const i = typeOptions.indexOf(mapping.type);
-    setChannelType(channel, typeOptions[(i + 1) % typeOptions.length]);
-  };
 
   const hasTransforms =
     !count &&
@@ -304,18 +314,20 @@ function ChannelPill({
   return (
     <div className={styles.pillWrap}>
       <div className={styles.pill}>
-        {/* TODO(ux-second-pass): the type chip cycles N→O→Q→T — no direct pick for
-            keyboard/SR users. Cycle vs. explicit radio is parked for a batched council
-            review (docs/ux-second-pass.md). */}
-        <button
-          type="button"
-          className={styles.pillType}
-          aria-label={`Field type: ${titleCase(currentType)}${canCycle ? ' — activate to change' : ''}`}
-          disabled={!canCycle}
-          onClick={cycleType}
-        >
-          {TYPE_ABBR[currentType]}
-        </button>
+        {/* The type chip opens a direct pick of the valid types (council 2026-06-12:
+            a cycling button gave keyboard/SR users no way to jump to a type). */}
+        <SelectControl
+          id={`cb-type-${channel}`}
+          label={`Field type for ${CHANNEL_LABELS[channel]}`}
+          heading="Field type"
+          options={typeOptions.map((t) => ({ value: t, label: titleCase(t) }))}
+          value={currentType}
+          onSelect={(t) => setChannelType(channel, t)}
+          triggerClassName={styles.pillType}
+          triggerContent={TYPE_ABBR[currentType]}
+          triggerTitle={titleCase(currentType)}
+          disabled={!canPick}
+        />
         <span className={styles.pillName} title={label}>
           {label}
         </span>
@@ -327,26 +339,25 @@ function ChannelPill({
       {hasTransforms && (
         <div className={styles.pillControls}>
           {supportsAggregate(mapping.type) && (
-            <label className={styles.transform}>
-              <span className={styles.miniLabel}>Aggregate</span>
-              <select
-                className={styles.mini}
+            <div className={styles.transform}>
+              <span className={styles.miniLabel} aria-hidden="true">
+                Aggregate
+              </span>
+              <SelectControl
+                id={`cb-agg-${channel}`}
+                label={`Aggregate for ${CHANNEL_LABELS[channel]}`}
+                heading="Aggregate"
+                options={[
+                  { value: '', label: 'None' },
+                  ...validAggregateOps(mapping.type).map((op) => ({
+                    value: op,
+                    label: AGGREGATE_LABELS[op],
+                  })),
+                ]}
                 value={mapping.aggregate && mapping.aggregate !== 'count' ? mapping.aggregate : ''}
-                onChange={(e) =>
-                  setChannelAggregate(
-                    channel,
-                    (e.target.value || undefined) as AggregateOp | undefined,
-                  )
-                }
-              >
-                <option value="">None</option>
-                {FIELD_AGGREGATES.map((op) => (
-                  <option key={op} value={op}>
-                    {titleCase(op)}
-                  </option>
-                ))}
-              </select>
-            </label>
+                onSelect={(v) => setChannelAggregate(channel, v || undefined)}
+              />
+            </div>
           )}
 
           {supportsBin(mapping.type) && (
@@ -361,23 +372,22 @@ function ChannelPill({
           )}
 
           {supportsTimeUnit(mapping.type) && (
-            <label className={styles.transform}>
-              <span className={styles.miniLabel}>Granularity</span>
-              <select
-                className={styles.mini}
+            <div className={styles.transform}>
+              <span className={styles.miniLabel} aria-hidden="true">
+                Granularity
+              </span>
+              <SelectControl
+                id={`cb-tu-${channel}`}
+                label={`Granularity for ${CHANNEL_LABELS[channel]}`}
+                heading="Granularity"
+                options={[
+                  { value: '', label: 'None (raw)' },
+                  ...TIME_UNITS.map((u) => ({ value: u, label: TIME_UNIT_LABELS[u] })),
+                ]}
                 value={mapping.timeUnit ?? ''}
-                onChange={(e) =>
-                  setChannelTimeUnit(channel, (e.target.value || undefined) as TimeUnit | undefined)
-                }
-              >
-                <option value="">None (raw)</option>
-                {TIME_UNITS.map((u) => (
-                  <option key={u} value={u}>
-                    {TIME_UNIT_LABELS[u]}
-                  </option>
-                ))}
-              </select>
-            </label>
+                onSelect={(v) => setChannelTimeUnit(channel, v || undefined)}
+              />
+            </div>
           )}
         </div>
       )}
@@ -421,36 +431,75 @@ function ChannelSlot({ channel, hint }: { channel: ChannelName; hint?: string })
         {active ? 'Pick a field…' : (hint ?? 'Add a field')}
       </button>
       {channelAcceptsValue(channel) && (
+        // A ghost button, not a link-styled affordance: it acts (binds a constant),
+        // and Carbon draws the line at links navigate / buttons act (council
+        // 2026-06-12). Verb-first label per the content rules.
         <button
           type="button"
           className={styles.slotConst}
           onClick={() => setChannelConstant(channel, String(defaultChannelValue(channel)))}
         >
-          or constant
+          Use a constant
         </button>
       )}
     </div>
   );
 }
 
+/** What a channel currently shows, for the assignment chooser's "replaces …" hint. */
+function occupantLabel(mapping: ChannelMapping | null | undefined): string | undefined {
+  if (!mapping) return undefined;
+  if (mapping.value !== undefined) return 'replaces the constant';
+  if (mapping.aggregate === 'count' && !mapping.field) return 'replaces Count';
+  return mapping.field ? `replaces ${mapping.field}` : undefined;
+}
+
+/** Channel names as the assignment chooser shows them (the on-chart shelf words). */
+const ASSIGN_LABELS: Record<ChannelName, string> = {
+  x: 'Columns (X)',
+  y: 'Rows (Y)',
+  color: 'Color',
+  size: 'Size',
+};
+
 /**
  * The field shelf (spec §06 → Encoding, field-first): the dataset's columns (plus any
  * calculated fields and a field-less "Count of records") as clickable chips with a type
- * glyph. Clicking a field assigns it to the armed channel, else the first empty channel
- * that accepts it (`assignField`). Past `FIELD_SHELF_SPLIT_MIN` columns it groups into
- * Dimensions (categories/dates) and Measures (numerics); a small dataset stays flat.
- * Already-mapped fields are dimmed (a field may still be placed on several channels).
+ * glyph. Clicking a chip opens an explicit channel chooser (the channels that accept the
+ * column; an occupied one says what it would replace); with a channel armed, the click
+ * assigns straight there instead and Esc disarms. Past `FIELD_SHELF_SPLIT_MIN` columns it
+ * groups into Dimensions (categories/dates) and Measures (numerics); a small dataset stays
+ * flat. Already-mapped fields are dimmed (a field may still be placed on several channels).
  */
 function FieldShelf() {
   const baseColumns = useChartBuilderStore((s) => s.columns);
   const calculates = useChartBuilderStore((s) => s.config.calculates);
   const encodings = useChartBuilderStore((s) => s.config.encodings);
   const assignField = useChartBuilderStore((s) => s.assignField);
+  const activeChannel = useChartBuilderStore((s) => s.activeChannel);
+  const focusChannel = useChartBuilderStore((s) => s.focusChannel);
   const columns = useMemo(
     () => effectiveColumns(baseColumns, calculates),
     [baseColumns, calculates],
   );
 
+  // Esc disarms the armed channel (captured so the modal itself doesn't close).
+  // TODO: with a channel armed AND a SelectControl popover open, one Esc both closes
+  // the popover and disarms — both are capture-phase document listeners, so
+  // stopPropagation can't serialize them. Rare combination; needs a shared
+  // escape-layer stack if it ever matters.
+  useEffect(() => {
+    if (!activeChannel) return;
+    const onKey = (e: KeyboardEvent) => {
+      if (e.key === 'Escape') {
+        e.stopPropagation();
+        focusChannel(null);
+      }
+    };
+    document.addEventListener('keydown', onKey, true);
+    return () => document.removeEventListener('keydown', onKey, true);
+  }, [activeChannel, focusChannel]);
+
   const assigned = useMemo(() => {
     const set = new Set<string>();
     for (const ch of CHANNELS) {
@@ -463,33 +512,59 @@ function FieldShelf() {
   const colTypeOf = (name: string): ColumnType =>
     columns.columnTypes.find((c) => c.name === name)?.type ?? 'string';
 
-  const fieldButton = (name: string) => (
-    <button
-      key={name}
-      type="button"
-      className={`${styles.shelfField} ${assigned.has(name) ? styles.shelfFieldUsed : ''}`}
-      onClick={() => assignField(name)}
-    >
-      <span className={styles.shelfGlyph} aria-hidden="true">
-        {TYPE_ABBR[defaultFieldType(colTypeOf(name))]}
-      </span>
-      <span className={styles.shelfFieldName}>{name}</span>
-    </button>
-  );
+  // A field chip: with a channel armed, clicking assigns straight there (the fast
+  // path); unarmed, it opens an explicit channel chooser instead of silently
+  // grabbing the first empty seat (council 2026-06-12 — NN/g #3, user choice).
+  const fieldChip = (
+    name: string,
+    display: string,
+    domId: string,
+    colType: ColumnType,
+    glyph: string,
+  ) => {
+    const choices = CHANNELS.filter((ch) => isColumnAllowedOnChannel(ch, colType));
+    return (
+      <SelectControl
+        key={domId}
+        id={`cb-assign-${domId}`}
+        label={`Add ${display} to a channel`}
+        heading="Add to"
+        options={choices.map((ch) => ({
+          value: ch,
+          label: ASSIGN_LABELS[ch],
+          detail: occupantLabel(encodings[ch]),
+        }))}
+        onSelect={(ch) => assignField(name, ch)}
+        beforeOpen={() => {
+          if (activeChannel) {
+            assignField(name);
+            return false;
+          }
+          return true;
+        }}
+        triggerClassName={`${styles.shelfField} ${assigned.has(name) ? styles.shelfFieldUsed : ''}`}
+        triggerContent={
+          <>
+            <span className={styles.shelfGlyph} aria-hidden="true">
+              {glyph}
+            </span>
+            <span className={styles.shelfFieldName}>{display}</span>
+          </>
+        }
+      />
+    );
+  };
 
-  const countButton = (
-    <button
-      key="__count"
-      type="button"
-      className={styles.shelfField}
-      onClick={() => assignField(COUNT_FIELD)}
-    >
-      <span className={styles.shelfGlyph} aria-hidden="true">
-        ∑
-      </span>
-      <span className={styles.shelfFieldName}>Count of records</span>
-    </button>
-  );
+  const fieldButton = (name: string) =>
+    fieldChip(
+      name,
+      name,
+      `f${columns.columns.indexOf(name)}`,
+      colTypeOf(name),
+      TYPE_ABBR[defaultFieldType(colTypeOf(name))],
+    );
+
+  const countButton = fieldChip(COUNT_FIELD, 'Count of records', 'count', 'number', '∑');
 
   const dimensions = columns.columns.filter((n) => colTypeOf(n) !== 'number');
   const measures = columns.columns.filter((n) => colTypeOf(n) === 'number');
@@ -499,22 +574,33 @@ function FieldShelf() {
   return (
     <div className={styles.fieldShelf}>
       <span className={styles.fieldLabel}>Fields</span>
-      {split ? (
-        <>
-          <span className={styles.shelfGroupHead}>Dimensions</span>
-          <div className={styles.shelfList}>{dimensions.map(fieldButton)}</div>
-          <span className={styles.shelfGroupHead}>Measures</span>
+      {/* Arming a channel must be visible at the place the next click happens
+          (NN/g #1): the shelf gains an accent ring and a status line naming the
+          target. The hint is a polite status so AT hears the mode change too. */}
+      {activeChannel && (
+        <p className={styles.armedHint} role="status">
+          Assigning to <strong>{CHANNEL_LABELS[activeChannel]}</strong> — choose a field below. Esc
+          cancels.
+        </p>
+      )}
+      <div className={`${styles.shelfScroll} ${activeChannel ? styles.shelfArmed : ''}`}>
+        {split ? (
+          <>
+            <span className={styles.shelfGroupHead}>Dimensions</span>
+            <div className={styles.shelfList}>{dimensions.map(fieldButton)}</div>
+            <span className={styles.shelfGroupHead}>Measures</span>
+            <div className={styles.shelfList}>
+              {measures.map(fieldButton)}
+              {countButton}
+            </div>
+          </>
+        ) : (
           <div className={styles.shelfList}>
-            {measures.map(fieldButton)}
+            {columns.columns.map(fieldButton)}
             {countButton}
           </div>
-        </>
-      ) : (
-        <div className={styles.shelfList}>
-          {columns.columns.map(fieldButton)}
-          {countButton}
-        </div>
-      )}
+        )}
+      </div>
     </div>
   );
 }
@@ -687,19 +773,15 @@ function FilterRow({ filter, columns }: { filter: BuilderFilter; columns: Builde
             onChange={(e) => updateFilter(filter.id, { expr: e.target.value })}
           />
         ) : (
-          <select
-            className={styles.filterField}
-            aria-label="Filter field"
-            value={filter.field ?? ''}
-            onChange={(e) => setFilterField(filter.id, e.target.value)}
-          >
-            {!filter.field && <option value="">Choose a field…</option>}
-            {columns.columns.map((name) => (
-              <option key={name} value={name}>
-                {name}
-              </option>
-            ))}
-          </select>
+          <SelectControl
+            id={`cb-ffield-${filter.id}`}
+            label="Filter field"
+            heading="Filter field"
+            options={columns.columns.map((name) => ({ value: name, label: name }))}
+            value={filter.field}
+            onSelect={(name) => setFilterField(filter.id, name)}
+            triggerContent={filter.field ? undefined : 'Choose a field…'}
+          />
         )}
         <button
           type="button"
@@ -713,18 +795,17 @@ function FilterRow({ filter, columns }: { filter: BuilderFilter; columns: Builde
 
       {!expressionMode && (
         <div className={styles.filterPredicate}>
-          <select
-            className={styles.mini}
-            aria-label="Filter operator"
+          <SelectControl
+            id={`cb-fop-${filter.id}`}
+            label="Filter operator"
+            heading="Operator"
+            options={validFilterOps(fieldType).map((o) => ({
+              value: o,
+              label: FILTER_OP_LABELS[o],
+            }))}
             value={op}
-            onChange={(e) => updateFilter(filter.id, { op: e.target.value as FilterOp })}
-          >
-            {validFilterOps(fieldType).map((o) => (
-              <option key={o} value={o}>
-                {FILTER_OP_LABELS[o]}
-              </option>
-            ))}
-          </select>
+            onSelect={(o) => updateFilter(filter.id, { op: o })}
+          />
           {arity === 'range' ? (
             <>
               <input
@@ -985,6 +1066,11 @@ function BuilderPreview() {
 
   const specText = useChartBuilderStore(selectBuilderSpecText);
   const valid = useChartBuilderStore(selectBuilderValid);
+  // An explicit Chart size must show up in the preview — the 'width' fit mode
+  // overwrites width AND drops height, so it only applies while sizing is auto.
+  const explicitSize = useChartBuilderStore(
+    (s) => s.config.width !== undefined || s.config.height !== undefined,
+  );
   const uiTheme = useAppStore((s) => s.uiTheme);
   const datasets = useDatasetStore(useShallow((s) => s.datasets));
 
@@ -1005,7 +1091,10 @@ function BuilderPreview() {
           const t0 = performance.now();
           const parsed: unknown = JSON.parse(specText);
           const t1 = performance.now();
-          const prepared = prepareSpecForRender(parsed, { fitMode: 'width', datasets });
+          const prepared = prepareSpecForRender(parsed, {
+            fitMode: explicitSize ? 'default' : 'width',
+            datasets,
+          });
           const t2 = performance.now();
           handleRef.current?.destroy(); // finalizing a huge prior SVG is itself a cost
           handleRef.current = null;
@@ -1069,7 +1158,7 @@ function BuilderPreview() {
     }, RENDER_DEBOUNCE_MS);
 
     return () => clearTimeout(timer);
-  }, [specText, valid, uiTheme, datasets]);
+  }, [specText, valid, explicitSize, uiTheme, datasets]);
 
   useEffect(
     () => () => {
@@ -1110,19 +1199,87 @@ function BuilderPreview() {
   );
 }
 
+/** A positive integer from a dimension input; blank/garbage reads as "auto". */
+function parseDim(raw: string): number | undefined {
+  if (raw.trim() === '') return undefined;
+  const n = Number(raw);
+  return Number.isFinite(n) && n > 0 ? Math.round(n) : undefined;
+}
+
+/**
+ * Chart properties — title/subtitle and explicit width/height, as a slim strip
+ * pinned under the preview: these describe *the chart*, so they live on the chart
+ * side, not in the encoding pane (council 2026-06-12 — NN/g #4, the Tableau/Lyra
+ * convention). The subtitle is disabled until a title exists because Vega-Lite has
+ * no standalone subtitle (it nests under `title`).
+ */
+function ChartProps() {
+  const title = useChartBuilderStore((s) => s.config.title ?? '');
+  const subtitle = useChartBuilderStore((s) => s.config.subtitle ?? '');
+  const width = useChartBuilderStore((s) => s.config.width);
+  const height = useChartBuilderStore((s) => s.config.height);
+  const setTitle = useChartBuilderStore((s) => s.setTitle);
+  const setSubtitle = useChartBuilderStore((s) => s.setSubtitle);
+  const setWidth = useChartBuilderStore((s) => s.setWidth);
+  const setHeight = useChartBuilderStore((s) => s.setHeight);
+  const hasTitle = title.trim() !== '';
+
+  return (
+    <div className={styles.chartProps}>
+      <label className={styles.propField}>
+        <span>Title</span>
+        <input
+          className={styles.propInput}
+          value={title}
+          placeholder="None"
+          onChange={(e) => setTitle(e.target.value)}
+        />
+      </label>
+      <label className={styles.propField}>
+        <span>Subtitle</span>
+        <input
+          className={styles.propInput}
+          value={subtitle}
+          placeholder={hasTitle ? 'None' : 'Add a title first'}
+          disabled={!hasTitle}
+          onChange={(e) => setSubtitle(e.target.value)}
+        />
+      </label>
+      <label className={styles.propField}>
+        <span>Width</span>
+        <input
+          type="number"
+          min={1}
+          className={styles.dimInput}
+          value={width ?? ''}
+          placeholder="auto"
+          onChange={(e) => setWidth(parseDim(e.target.value))}
+        />
+      </label>
+      <label className={styles.propField}>
+        <span>Height</span>
+        <input
+          type="number"
+          min={1}
+          className={styles.dimInput}
+          value={height ?? ''}
+          placeholder="auto"
+          onChange={(e) => setHeight(parseDim(e.target.value))}
+        />
+      </label>
+    </div>
+  );
+}
+
 export function ChartBuilderModal() {
   const datasetId = useChartBuilderStore((s) => s.datasetId);
   const datasetName = useChartBuilderStore((s) => s.config.datasetName);
   const mark = useChartBuilderStore((s) => s.config.mark);
-  const width = useChartBuilderStore((s) => s.config.width);
-  const height = useChartBuilderStore((s) => s.config.height);
   const sort = useChartBuilderStore((s) => s.config.sort);
   const stack = useChartBuilderStore((s) => s.config.stack);
   const setMark = useChartBuilderStore((s) => s.setMark);
   const setSort = useChartBuilderStore((s) => s.setSort);
   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);
 
@@ -1170,12 +1327,6 @@ export function ChartBuilderModal() {
     return <p className={styles.muted}>No dataset loaded. Open this from a dataset in Datasets.</p>;
   }
 
-  const parseDim = (raw: string): number | undefined => {
-    if (raw.trim() === '') return undefined;
-    const n = Number(raw);
-    return Number.isFinite(n) && n > 0 ? Math.round(n) : undefined;
-  };
-
   return (
     <div className={styles.builder}>
       <div className={styles.configPane} ref={configPaneRef} tabIndex={-1}>
@@ -1228,36 +1379,6 @@ export function ChartBuilderModal() {
           </div>
         )}
 
-        <div className={styles.dimensions}>
-          {/* "Chart size", not just "Size" — the Marks card now has a Size *encoding*
-              channel; this is the rendered chart's width/height. */}
-          <span className={styles.fieldLabel}>Chart size (optional)</span>
-          <div className={styles.dimInputs}>
-            <label className={styles.dimField}>
-              <span>Width</span>
-              <input
-                type="number"
-                min={1}
-                className={styles.dimInput}
-                value={width ?? ''}
-                placeholder="auto"
-                onChange={(e) => setWidth(parseDim(e.target.value))}
-              />
-            </label>
-            <label className={styles.dimField}>
-              <span>Height</span>
-              <input
-                type="number"
-                min={1}
-                className={styles.dimInput}
-                value={height ?? ''}
-                placeholder="auto"
-                onChange={(e) => setHeight(parseDim(e.target.value))}
-              />
-            </label>
-          </div>
-        </div>
-
         {warnings.length > 0 && (
           <ul
             className={styles.warnings}
@@ -1314,6 +1435,7 @@ export function ChartBuilderModal() {
       <div className={styles.previewSide}>
         <OnChartShelves />
         <BuilderPreview />
+        <ChartProps />
       </div>
     </div>
   );

src/app/components/SelectControl.module.css

@@ -0,0 +1,123 @@
+/* SelectControl — the app's value-picker disclosure (replaces native <select>;
+   arch 10 §5). Trigger + panel mirror SortControl's geometry and tokens. */
+
+.trigger {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--space-1);
+  padding: var(--space-1) var(--space-2);
+  border: var(--border-width) solid var(--border-strong);
+  border-radius: var(--radius);
+  background: var(--bg);
+  color: var(--text);
+  font: inherit;
+  font-size: 12px;
+  white-space: nowrap;
+  cursor: pointer;
+  transition:
+    background var(--dur-fast) var(--ease),
+    color var(--dur-fast) var(--ease);
+}
+
+.trigger:hover:not(:disabled) {
+  background: var(--layer-01);
+}
+
+.trigger[aria-expanded='true'] {
+  background: var(--layer-02);
+}
+
+.trigger:disabled {
+  cursor: default;
+  opacity: 0.6;
+}
+
+.trigger:focus-visible {
+  outline: 2px solid var(--focus);
+  outline-offset: 1px;
+}
+
+.triggerLabel {
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.caret {
+  font-size: 9px;
+  color: var(--text-secondary);
+}
+
+/* The disclosed panel — portaled to <body>, fixed, above modal content. */
+.pop {
+  position: fixed;
+  z-index: 2000;
+  min-width: 150px;
+  max-width: 320px;
+  padding: var(--space-2);
+  background: var(--layer-01);
+  border: var(--border-width) solid var(--border-strong);
+  border-radius: var(--radius);
+  box-shadow: 0 8px 24px rgb(0 0 0 / 0.4);
+}
+
+.heading {
+  margin: 0 0 var(--space-2);
+  padding: 0 var(--space-2);
+  font-size: 11px;
+  font-weight: 600;
+  letter-spacing: 0.06em;
+  text-transform: uppercase;
+  color: var(--text-secondary);
+}
+
+/* A long option list (a 25-column filter-field picker) scrolls in place. */
+.list {
+  display: flex;
+  flex-direction: column;
+  max-height: min(320px, 50vh);
+  overflow-y: auto;
+}
+
+.option {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: var(--space-3);
+  width: 100%;
+  padding: var(--space-2) var(--space-3);
+  border: none;
+  border-radius: var(--radius);
+  background: transparent;
+  color: var(--text);
+  font: inherit;
+  font-size: 13px;
+  text-align: left;
+  cursor: pointer;
+  transition: background var(--dur-fast) var(--ease);
+}
+
+.option:hover {
+  background: var(--layer-02);
+}
+
+.option:focus-visible {
+  outline: 2px solid var(--focus);
+  outline-offset: -1px;
+}
+
+.selected {
+  font-weight: 600;
+}
+
+.optionLabel {
+  display: flex;
+  flex-direction: column;
+  min-width: 0;
+}
+
+.detail {
+  font-size: 11px;
+  font-weight: 400;
+  color: var(--text-secondary);
+}

src/app/components/SelectControl.tsx

@@ -0,0 +1,243 @@
+/**
+ * SelectControl — the app's value picker: a disclosure trigger showing the current
+ * choice plus a portaled, non-modal popover listing the options as plain buttons.
+ *
+ * This replaces native `<select>` elements wherever a control is part of a designed
+ * surface: a native select's popup can't be token-styled and renders differently on
+ * every browser/OS, which reads as a foreign object inside an otherwise consistent
+ * UI (arch 10 §5 records the resolution). It is the same APG **disclosure**
+ * primitive as SortControl/SettingsPopover — deliberately NOT an ARIA menu and not
+ * a combobox; a short list of buttons needs neither's contract.
+ *
+ * Behaviour (mirrors SortControl): at most one popover is open app-wide
+ * (`useSettingsPopoverStore`); Esc closes and refocuses the trigger; an outside
+ * pointer press closes; opening focuses the selected option (or the first);
+ * Arrow/Home/End move focus through the options; choosing one fires `onSelect`,
+ * closes, and refocuses the trigger. The panel is portaled to <body> and positioned
+ * fixed so it escapes pane/modal overflow clipping, flipping above the trigger when
+ * the viewport below is too short.
+ *
+ * The same control doubles as an **action picker** (e.g. "Add field to which
+ * channel?"): pass no `value` and a custom `triggerContent`; `beforeOpen` lets the
+ * caller intercept the click entirely (the armed-channel fast path).
+ */
+
+import { useCallback, useEffect, useRef, type ReactNode } from 'react';
+import { createPortal } from 'react-dom';
+import { useSettingsPopoverStore } from '../stores/SettingsPopoverStore';
+import styles from './SelectControl.module.css';
+
+/** Gap (px) between the trigger and the disclosed panel (matches SortControl). */
+const GAP = 6;
+
+export interface SelectControlOption<V extends string> {
+  value: V;
+  label: string;
+  /** Optional secondary line (e.g. "replaces Ship Mode" on an occupied channel). */
+  detail?: string;
+}
+
+export interface SelectControlProps<V extends string> {
+  /** Unique id — popover registry key and the panel's DOM id. */
+  id: string;
+  /** Accessible name for the control ("Aggregate for Y", "Add Region to…"). */
+  label: string;
+  options: ReadonlyArray<SelectControlOption<V>>;
+  /** Current value; omit for an action picker (no option reads as selected). */
+  value?: V;
+  onSelect: (value: V) => void;
+  /** Trigger body; defaults to the current option's label plus a caret. */
+  triggerContent?: ReactNode;
+  /** Replaces (not extends) the default trigger styling — for chip-styled triggers. */
+  triggerClassName?: string;
+  triggerTitle?: string;
+  disabled?: boolean;
+  /** Popover heading; defaults to `label`. */
+  heading?: string;
+  /** Return false to swallow the trigger click without opening (fast paths). */
+  beforeOpen?: () => boolean;
+}
+
+export function SelectControl<V extends string>({
+  id,
+  label,
+  options,
+  value,
+  onSelect,
+  triggerContent,
+  triggerClassName,
+  triggerTitle,
+  disabled,
+  heading,
+  beforeOpen,
+}: SelectControlProps<V>) {
+  const open = useSettingsPopoverStore((s) => s.openId === id);
+  const toggle = useSettingsPopoverStore((s) => s.toggle);
+  const close = useSettingsPopoverStore((s) => s.close);
+  const triggerRef = useRef<HTMLButtonElement>(null);
+  const popRef = useRef<HTMLDivElement | null>(null);
+
+  const current = value !== undefined ? options.find((o) => o.value === value) : undefined;
+
+  // Fixed-position from the trigger's rect (no React state → no re-render on
+  // scroll). Below the trigger by default; above when the viewport below is short.
+  const place = useCallback(() => {
+    const trigger = triggerRef.current;
+    const pop = popRef.current;
+    if (!trigger || !pop) return;
+    const r = trigger.getBoundingClientRect();
+    const below = window.innerHeight - r.bottom - GAP;
+    const height = pop.offsetHeight;
+    pop.style.top =
+      below < height && r.top > height + GAP ? `${r.top - GAP - height}px` : `${r.bottom + GAP}px`;
+    // Keep the panel on-screen when the trigger sits near the right edge.
+    const left = Math.min(r.left, window.innerWidth - pop.offsetWidth - GAP);
+    pop.style.left = `${Math.max(GAP, left)}px`;
+  }, []);
+
+  useEffect(() => {
+    if (!open) return;
+    window.addEventListener('resize', place);
+    window.addEventListener('scroll', place, true);
+    return () => {
+      window.removeEventListener('resize', place);
+      window.removeEventListener('scroll', place, true);
+    };
+  }, [open, place]);
+
+  // Esc closes + restores focus to the trigger; an outside pointer press closes
+  // (APG disclosure; non-modal). Esc is captured so it settles here, not on a
+  // parent (the builder modal also listens for Esc).
+  useEffect(() => {
+    if (!open) return;
+    const onKey = (e: KeyboardEvent) => {
+      if (e.key === 'Escape') {
+        e.stopPropagation();
+        close();
+        triggerRef.current?.focus();
+      }
+    };
+    const onPointer = (e: PointerEvent) => {
+      const t = e.target as Node;
+      if (!popRef.current?.contains(t) && !triggerRef.current?.contains(t)) close();
+    };
+    document.addEventListener('keydown', onKey, true);
+    document.addEventListener('pointerdown', onPointer, true);
+    return () => {
+      document.removeEventListener('keydown', onKey, true);
+      document.removeEventListener('pointerdown', onPointer, true);
+    };
+  }, [open, close]);
+
+  // Arrow/Home/End roving among the option buttons — a convenience on top of the
+  // natural Tab order, matching what a native select's popup offers.
+  const onPopKeyDown = (e: React.KeyboardEvent) => {
+    const pop = popRef.current;
+    if (!pop) return;
+    // Tab closes the popup and resumes tabbing from the trigger (the native-select
+    // convention) — also keeps focus inside a host modal's trap, since the panel is
+    // portaled outside it.
+    if (e.key === 'Tab') {
+      close();
+      triggerRef.current?.focus();
+      return;
+    }
+    const items = Array.from(pop.querySelectorAll<HTMLButtonElement>('button'));
+    const i = items.indexOf(document.activeElement as HTMLButtonElement);
+    let next = -1;
+    if (e.key === 'ArrowDown') next = i < 0 ? 0 : Math.min(i + 1, items.length - 1);
+    else if (e.key === 'ArrowUp') next = i < 0 ? items.length - 1 : Math.max(i - 1, 0);
+    else if (e.key === 'Home') next = 0;
+    else if (e.key === 'End') next = items.length - 1;
+    if (next >= 0) {
+      e.preventDefault();
+      items[next]?.focus();
+    }
+  };
+
+  // On mount: position before paint, then land focus on the selected option (or
+  // the first) so keyboard users arrive inside the popover.
+  const setPopNode = useCallback(
+    (node: HTMLDivElement | null) => {
+      popRef.current = node;
+      if (node) {
+        place();
+        // Two queries, not one selector list — `querySelector('a, b')` returns the
+        // first match in document order, which would always be the first button.
+        const target =
+          node.querySelector<HTMLElement>('[aria-current="true"]') ??
+          node.querySelector<HTMLElement>('button');
+        target?.focus();
+      }
+    },
+    [place],
+  );
+
+  const choose = (v: V) => {
+    onSelect(v);
+    close();
+    triggerRef.current?.focus();
+  };
+
+  return (
+    <>
+      <button
+        ref={triggerRef}
+        type="button"
+        className={triggerClassName ?? styles.trigger}
+        aria-expanded={open}
+        aria-controls={open ? id : undefined}
+        aria-label={current ? `${label}: ${current.label}` : label}
+        title={triggerTitle}
+        disabled={disabled}
+        onClick={() => {
+          if (beforeOpen && !beforeOpen()) return;
+          toggle(id);
+        }}
+      >
+        {triggerContent ?? (
+          <>
+            <span className={styles.triggerLabel}>{current?.label ?? '—'}</span>
+            <span className={styles.caret} aria-hidden="true">
+              ▾
+            </span>
+          </>
+        )}
+      </button>
+      {open &&
+        createPortal(
+          <div
+            ref={setPopNode}
+            id={id}
+            className={styles.pop}
+            role="group"
+            aria-label={label}
+            onKeyDown={onPopKeyDown}
+          >
+            <h4 className={styles.heading}>{heading ?? label}</h4>
+            <div className={styles.list}>
+              {options.map((o) => {
+                const selected = value !== undefined && o.value === value;
+                return (
+                  <button
+                    key={o.value}
+                    type="button"
+                    className={`${styles.option} ${selected ? styles.selected : ''}`}
+                    aria-current={selected || undefined}
+                    onClick={() => choose(o.value)}
+                  >
+                    <span className={styles.optionLabel}>
+                      {o.label}
+                      {o.detail !== undefined && <span className={styles.detail}>{o.detail}</span>}
+                    </span>
+                    {selected && <span aria-hidden="true">✓</span>}
+                  </button>
+                );
+              })}
+            </div>
+          </div>,
+          document.body,
+        )}
+    </>
+  );
+}

src/app/stores/ChartBuilderStore.test.ts

@@ -104,6 +104,38 @@ describe('transforms — aggregate / bin / timeUnit / count', () => {
     expect(cb().config.encodings.x).toEqual({ field: 'price', type: 'nominal' });
   });
 
+  test('assignField with an explicit channel places the field there, replacing any occupant', () => {
+    const id = seedDataset('S', [{ region: 'N', revenue: 5 }]);
+    cb().init(id);
+    cb().setChannelColumn('x', 'revenue');
+    cb().assignField('region', 'x'); // explicit target from the chip's channel chooser
+    expect(cb().config.encodings.x).toEqual({ field: 'region', type: 'nominal' });
+    expect(cb().activeChannel).toBeNull();
+  });
+
+  test('setTitle / setSubtitle set and clear the chart title fields', () => {
+    const id = seedDataset('S', [{ v: 1 }]);
+    cb().init(id);
+    cb().setTitle('My chart');
+    cb().setSubtitle('A closer look');
+    expect(cb().config.title).toBe('My chart');
+    expect(cb().config.subtitle).toBe('A closer look');
+    cb().setTitle('');
+    expect(cb().config.title).toBeUndefined();
+  });
+
+  test('retyping drops a now-invalid aggregate but keeps a distinct (valid for any type)', () => {
+    const id = seedDataset('S', [{ price: 5 }]);
+    cb().init(id);
+    cb().setChannelColumn('x', 'price');
+    cb().setChannelAggregate('x', 'sum');
+    cb().setChannelType('x', 'nominal'); // sum can't reduce a nominal field
+    expect(cb().config.encodings.x?.aggregate).toBeUndefined();
+    cb().setChannelAggregate('x', 'distinct');
+    cb().setChannelType('x', 'ordinal'); // distinct counts unique values of anything
+    expect(cb().config.encodings.x?.aggregate).toBe('distinct');
+  });
+
   test('setChannelTimeUnit sets and clears granularity', () => {
     const id = seedDataset('S', [{ day: '2026-01-01', v: 1 }]);
     cb().init(id);

src/app/stores/ChartBuilderStore.ts

@@ -27,9 +27,9 @@ import {
   isChannelTypeAllowed,
   isColumnAllowedOnChannel,
   pruneEncodings,
-  supportsAggregate,
   supportsBin,
   supportsTimeUnit,
+  validAggregateOps,
   validFieldTypes,
   validFilterOps,
   type AggregateOp,
@@ -106,9 +106,11 @@ export interface ChartBuilderState {
   /**
    * Place a clicked shelf field on a channel: the armed channel if it accepts the
    * column, else the first empty channel that does (X→Y→Color→Size). A no-op when no
-   * channel can take it. Clears the armed slot.
+   * channel can take it. Clears the armed slot. An explicit `channel` (the field
+   * chip's channel chooser) overrides both — it places the column there even when
+   * the channel is occupied (the chooser labels that as a replacement).
    */
-  assignField: (columnName: string) => void;
+  assignField: (columnName: string, channel?: ChannelName) => void;
   /**
    * Set/replace a channel's binding with a **constant value** (the Property model,
    * spec §06) — a fixed colour/size emitted as Vega-Lite `{ value }`. Only Color/Size
@@ -151,6 +153,10 @@ export interface ChartBuilderState {
   removeCalculate: (id: string) => void;
   setWidth: (width: number | undefined) => void;
   setHeight: (height: number | undefined) => void;
+  /** Set/clear the chart title (whitespace-only clears). */
+  setTitle: (raw: string) => void;
+  /** Set/clear the subtitle; emitted into the spec only alongside a title. */
+  setSubtitle: (raw: string) => void;
   /** Build the spec, create + activate a linked snippet, toast, and close. */
   createSnippet: (now?: Date) => boolean;
   reset: () => void;
@@ -259,9 +265,9 @@ export const useChartBuilderStore = create<ChartBuilderState>((set, get) => ({
 
   focusChannel: (channel) => set({ activeChannel: channel }),
 
-  assignField: (columnName) =>
+  assignField: (columnName, channel) =>
     set((s) => {
-      const target = chooseTargetChannel(s, columnName);
+      const target = channel ?? chooseTargetChannel(s, columnName);
       if (!target) return { activeChannel: null }; // nothing can take it → just disarm
       const mapping = mappingForColumn(s, target, columnName);
       return {
@@ -288,10 +294,18 @@ export const useChartBuilderStore = create<ChartBuilderState>((set, get) => ({
     set((s) => {
       const current = s.config.encodings[channel];
       if (!current) return s; // no field on this channel → nothing to retype
-      // Drop transforms that no longer apply to the new type (e.g. an aggregate or
-      // bin when leaving Quantitative, a granularity when leaving Temporal).
+      // Drop transforms that no longer apply to the new type (e.g. a sum or bin when
+      // leaving Quantitative, a granularity when leaving Temporal). A `distinct`
+      // survives any retype (it applies to every field type); `count` is field-less
+      // and never reaches here (a count pill offers no type cycle).
       const next: ChannelMapping = { ...current, type };
-      if (!supportsAggregate(type)) delete next.aggregate;
+      if (
+        next.aggregate &&
+        next.aggregate !== 'count' &&
+        !validAggregateOps(type).includes(next.aggregate)
+      ) {
+        delete next.aggregate;
+      }
       if (!supportsBin(type)) delete next.bin;
       if (!supportsTimeUnit(type)) delete next.timeUnit;
       return updateEncoding(s, channel, next);
@@ -451,6 +465,11 @@ export const useChartBuilderStore = create<ChartBuilderState>((set, get) => ({
   setWidth: (width) => set((s) => ({ config: { ...s.config, width } })),
   setHeight: (height) => set((s) => ({ config: { ...s.config, height } })),
 
+  // The raw input value is kept (mid-edit spaces and all) so typing feels normal;
+  // the assembler trims and skips a blank/whitespace-only title at emission.
+  setTitle: (raw) => set((s) => ({ config: { ...s.config, title: raw || undefined } })),
+  setSubtitle: (raw) => set((s) => ({ config: { ...s.config, subtitle: raw || undefined } })),
+
   createSnippet: (now) => {
     const { config } = get();
     if (!isBuilderConfigValid(config)) return false; // guarded by a disabled action too