fix(chart): guard empty data array and fix X-axis cross-filter visual feedback

- Add null check on `props.data[0]` in click and context menu handlers
  to prevent undefined access when data array is empty (Copilot #2, #3)
- Add `hasDimensions` option to `transformSeries` so series-level
  opacity dimming is skipped when cross-filtering by X-axis value,
  since `selectedValues` contains X-axis values rather than series
  names in that case (Copilot #4)
- Add tests for both dimension-based and X-axis-based filtering opacity

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rusackas

superset-frontend/plugins/plugin-chart-echarts/src/Timeseries/EchartsTimeseries.tsx

@@ -231,10 +231,9 @@ export default function EchartsTimeseries({
           // Cross-filter by dimension (original behavior)
           const { seriesName: name } = props;
           handleChange(name);
-        } else if (canCrossFilterByXAxis && props.data) {
+        } else if (canCrossFilterByXAxis && props.data?.[0] != null) {
           // Cross-filter by X-axis value when no dimensions (issue #25334)
-          const xAxisValue = props.data[0];
-          handleXAxisChange(xAxisValue);
+          handleXAxisChange(props.data[0]);
         }
       }, TIMER_DURATION);
     },
@@ -316,7 +315,7 @@ export default function EchartsTimeseries({
         let crossFilter;
         if (hasDimensions) {
           crossFilter = getCrossFilterDataMask(seriesName);
-        } else if (canCrossFilterByXAxis && data) {
+        } else if (canCrossFilterByXAxis && data?.[0] != null) {
           crossFilter = getXAxisCrossFilterDataMask(data[0]);
         }
 

superset-frontend/plugins/plugin-chart-echarts/src/Timeseries/transformProps.ts

@@ -419,6 +419,7 @@ export default function transformProps(
         timeCompare: array,
         timeShiftColor,
         theme,
+        hasDimensions: groupBy.length > 0,
       },
     );
     if (transformedSeries) {

superset-frontend/plugins/plugin-chart-echarts/src/Timeseries/transformers.ts

@@ -196,6 +196,7 @@ export function transformSeries(
     timeCompare?: string[];
     timeShiftColor?: boolean;
     theme?: SupersetTheme;
+    hasDimensions?: boolean;
   },
 ): SeriesOption | undefined {
   const { name, data } = series;
@@ -237,8 +238,12 @@ export function transformSeries(
   const isConfidenceBand =
     forecastSeries.type === ForecastSeriesEnum.ForecastLower ||
     forecastSeries.type === ForecastSeriesEnum.ForecastUpper;
+  // When cross-filtering by X-axis (no dimensions), selectedValues contains
+  // X-axis values rather than series names, so skip series-level dimming.
   const isFiltered =
-    filterState?.selectedValues && !filterState?.selectedValues.includes(name);
+    opts.hasDimensions !== false &&
+    filterState?.selectedValues &&
+    !filterState?.selectedValues.includes(name);
   const opacity = isFiltered
     ? OpacityEnum.SemiTransparent
     : opts.lineStyle?.opacity || OpacityEnum.NonTransparent;

superset-frontend/plugins/plugin-chart-echarts/test/Timeseries/transformers.test.ts

@@ -89,6 +89,34 @@ describe('transformSeries', () => {
     expect((result as any).itemStyle.borderType).toBeUndefined();
     expect((result as any).itemStyle.borderColor).toBeUndefined();
   });
+
+  it('should dim series when selectedValues does not include series name (dimension-based filtering)', () => {
+    const opts = {
+      filterState: { selectedValues: ['other-series'] },
+      hasDimensions: true,
+      seriesType: EchartsTimeseriesSeriesType.Bar,
+      timeShiftColor: false,
+    };
+
+    const result = transformSeries(series, mockColorScale, 'test-key', opts);
+
+    // OpacityEnum.SemiTransparent = 0.3
+    expect((result as any).itemStyle.opacity).toBe(0.3);
+  });
+
+  it('should not dim series when hasDimensions is false (X-axis cross-filtering)', () => {
+    const opts = {
+      filterState: { selectedValues: ['Product A'] },
+      hasDimensions: false,
+      seriesType: EchartsTimeseriesSeriesType.Bar,
+      timeShiftColor: false,
+    };
+
+    const result = transformSeries(series, mockColorScale, 'test-key', opts);
+
+    // OpacityEnum.NonTransparent = 1 (not dimmed)
+    expect((result as any).itemStyle.opacity).toBe(1);
+  });
 });
 
 describe('transformNegativeLabelsPosition', () => {