fix(echarts): Show full labels in bar chart tooltips

Fixes #31864

When bar chart axis labels are visually truncated by ECharts due to space
constraints, the tooltip now shows the full label text by preferring the
axisValue/axisValueLabel properties which contain the complete text.

This allows users to hover over data points and see the full name even when
the axis label is shortened for display purposes.

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

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

@@ -1010,8 +1010,12 @@ export default function transformProps(
       trigger: richTooltip ? 'axis' : 'item',
       formatter: (params: any) => {
         const [xIndex, yIndex] = isHorizontal ? [1, 0] : [0, 1];
+        // For axis tooltips, prefer axisValue/axisValueLabel which contains the full label
+        // even when the axis label is visually truncated
         const xValue: number = richTooltip
-          ? params[0].value[xIndex]
+          ? (params[0].axisValue ??
+            params[0].axisValueLabel ??
+            params[0].value[xIndex])
           : params.value[xIndex];
         const forecastValue: CallbackDataParams[] = richTooltip
           ? params

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

@@ -1520,3 +1520,99 @@ test('should assign distinct dash patterns for multiple time offsets consistentl
   // must be different patterns
   expect(symbol1).not.toEqual(symbol2);
 });
+
+describe('Tooltip with long labels', () => {
+  test('should use axisValue for tooltip when available (richTooltip)', () => {
+    const longLabelData: ChartDataResponseResult[] = [
+      createTestQueryData([
+        {
+          'This is a very long category name that would normally be truncated': 100,
+          __timestamp: 599616000000,
+        },
+        {
+          'Another extremely long category name for testing purposes': 200,
+          __timestamp: 599916000000,
+        },
+      ]),
+    ];
+
+    const chartProps = createTestChartProps({
+      formData: {
+        richTooltip: true,
+      },
+      queriesData: longLabelData,
+    });
+
+    const transformedProps = transformProps(chartProps);
+
+    // Get the tooltip formatter function
+    const tooltipFormatter = (transformedProps.echartOptions as any).tooltip
+      .formatter;
+
+    // Simulate params from ECharts with axisValue containing full label
+    // Use distinct values for axisValue and seriesName to verify axisValue is used
+    const mockParams = [
+      {
+        axisValue:
+          'This is a very long category name that would normally be truncated',
+        value: [599616000000, 100],
+        seriesName: 'Some Series Name',
+      },
+    ];
+
+    // Call the formatter and check it uses the full label from axisValue
+    const result = tooltipFormatter(mockParams);
+    expect(result).toContain(
+      'This is a very long category name that would normally be truncated',
+    );
+  });
+
+  test('should fallback to value when axisValue is not available', () => {
+    const chartProps = createTestChartProps({
+      formData: {
+        richTooltip: true,
+      },
+    });
+
+    const transformedProps = transformProps(chartProps);
+
+    const tooltipFormatter = (transformedProps.echartOptions as any).tooltip
+      .formatter;
+
+    // Simulate params without axisValue
+    const mockParams = [
+      {
+        value: [599616000000, 1],
+        seriesName: 'San Francisco',
+      },
+    ];
+
+    // Should still work with fallback to value
+    const result = tooltipFormatter(mockParams);
+    expect(result).toBeDefined();
+    expect(typeof result).toBe('string');
+  });
+
+  test('should handle item tooltips correctly', () => {
+    const chartProps = createTestChartProps({
+      formData: {
+        richTooltip: false,
+      },
+    });
+
+    const transformedProps = transformProps(chartProps);
+
+    const tooltipFormatter = (transformedProps.echartOptions as any).tooltip
+      .formatter;
+
+    // For item tooltips, params is a single object
+    const mockParams = {
+      value: [599616000000, 1],
+      seriesName: 'San Francisco',
+    };
+
+    const result = tooltipFormatter(mockParams);
+    expect(result).toBeDefined();
+    expect(typeof result).toBe('string');
+  });
+});