fix(ChartState): Don't create spurious implicit series when mark accessor matches chart's own axis accessor, fixing domain corruption for heatmap/Cell charts

Author: techniq

.changeset/fix-implicit-series-domain.md

@@ -0,0 +1,5 @@
+---
+'layerchart': patch
+---
+
+fix(ChartState): Don't create spurious implicit series when mark accessor matches chart's own axis accessor, fixing domain corruption for heatmap/Cell charts

packages/layerchart/src/lib/states/__fixtures__/ComponentNodeLifecycleChild.svelte

@@ -6,7 +6,7 @@
   ctx.registerComponent({
     name: 'ComponentNodeLifecycleChild',
     kind: 'mark',
-    markInfo: () => ({ y: 'value' }),
+    markInfo: () => ({ y: 'other' }),
   });
 </script>
 

packages/layerchart/src/lib/states/chart.svelte.test.ts

@@ -225,7 +225,6 @@ describe('ChartState mark registration', () => {
     const { state, cleanup } = createChartState<TestData>({
       data: [{ date: '2024-01', value: 10 }],
       x: 'date',
-      y: 'value',
     });
 
     try {
@@ -250,6 +249,24 @@ describe('ChartState mark registration', () => {
     }
   });
 
+  it('should not create implicit series when mark accessor matches chart accessor', () => {
+    const { state, cleanup } = createChartState<TestData>({
+      data: [{ date: '2024-01', value: 10 }],
+      x: 'date',
+      y: 'value',
+    });
+
+    try {
+      // Mark with same y as chart — not a new series, just using chart's axis
+      state.registerMark({ y: 'value', color: 'red' });
+      flushSync();
+
+      expect(state.seriesState.isDefaultSeries).toBe(true);
+    } finally {
+      cleanup();
+    }
+  });
+
   it('should generate implicit series from marks with string y accessors', () => {
     const data: MultiSeriesData[] = [
       { date: '2024-01', apples: 10, bananas: 15 },

packages/layerchart/src/lib/states/chart.svelte.ts

@@ -250,14 +250,19 @@ export class ChartState<
 
         // Generate implicit series from registered marks.
         // Use the value axis accessor (y for horizontal charts, x for vertical).
-        const valueAxis = this.props.valueAxis ?? 'y';
+        const valueAxis = this.valueAxis;
+        const chartValueProp = valueAxis === 'y' ? this.props.y : this.props.x;
         const implicitSeries: SeriesData<TData, any>[] = [];
         for (const { info } of this._markInfos) {
           const valueAccessor = valueAxis === 'y' ? info.y : info.x;
           const key =
             info.seriesKey ??
             (typeof valueAccessor === 'string' ? (valueAccessor as string) : undefined);
           if (!key) continue;
+          // Skip if the mark just reuses the chart's own axis accessor and has no
+          // separate data — it's not defining a new series, just using the chart's axis.
+          // Marks with their own data arrays are kept (multi-dataset scenario).
+          if (key === chartValueProp && !info.data) continue;
           if (implicitSeries.some((s) => s.key === key)) continue;
           implicitSeries.push({
             key,