fix: clarify Y axis clamping comment covers derived bounds too

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -724,6 +724,41 @@ export default function transformProps(
     }
   }
 
+  // Whenever a Y axis bound is defined, whether explicitly configured or
+  // derived above from the data, clamp series values to those bounds
+  // instead of leaving raw out-of-range values in place. ECharts axis
+  // clipping can otherwise drop an out-of-bounds point (and the line
+  // segments around it) entirely rather than truncating it at the
+  // boundary (see https://github.com/apache/superset/issues/27449).
+  if (yAxisMin !== undefined || yAxisMax !== undefined) {
+    const valueIndex = isHorizontal ? 0 : 1;
+    const clampAxisValue = (
+      value: string | number | null | undefined,
+    ): string | number | null | undefined => {
+      if (typeof value !== 'number' || Number.isNaN(value)) return value;
+      let clamped = value;
+      if (yAxisMin !== undefined) clamped = Math.max(clamped, yAxisMin);
+      if (yAxisMax !== undefined) clamped = Math.min(clamped, yAxisMax);
+      return clamped;
+    };
+    series.forEach(s => {
+      if (!Array.isArray(s.data)) return;
+      const clampedData = (
+        s.data as (string | number | null | undefined)[][]
+      ).map(point => {
+        if (Array.isArray(point)) {
+          const newPoint = [...point];
+          newPoint[valueIndex] = clampAxisValue(newPoint[valueIndex]);
+          return newPoint;
+        }
+        return point;
+      });
+      // Matches the existing pattern used elsewhere in this file for
+      // narrowing the broad, union-typed ECharts `SeriesOption.data` field.
+      (s as any).data = clampedData;
+    });
+  }
+
   // A dashboard-level time grain override (e.g. via a filter or the temporal
   // range control) is delivered in extraFormData and should take precedence
   // over the chart's own time grain when formatting temporal axes/tooltips.

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

@@ -1383,6 +1383,78 @@ test('should not apply axis bounds calculation when seriesType is not Bar for ho
   expect(xAxisRaw.max).toBeUndefined();
 });
 
+test('clamps series values to the yAxis max instead of dropping out-of-range points (#27449)', () => {
+  const queriesData: ChartDataResponseResult[] = [
+    createTestQueryData(
+      createTestData(
+        [
+          { 'Series A': 1 },
+          { 'Series A': 2 },
+          { 'Series A': 3 },
+          { 'Series A': 4 },
+          { 'Series A': 1000 },
+          { 'Series A': 4 },
+          { 'Series A': 2 },
+        ],
+        { intervalMs: 300000000 },
+      ),
+    ),
+  ];
+
+  const chartProps = createTestChartProps({
+    formData: {
+      ...formData,
+      groupby: [],
+      seriesType: EchartsTimeseriesSeriesType.Line,
+      truncateYAxis: true,
+      yAxisBounds: [0, 10],
+    },
+    queriesData,
+  });
+
+  const transformedProps = transformProps(chartProps);
+  const series = transformedProps.echartOptions.series as SeriesOption[];
+  const seriesA = series.find(s => s.name === 'Series A');
+  expect(seriesA).toBeDefined();
+  const data = seriesA!.data as [number, number][];
+
+  // The point that was 1000 should be present (not dropped) and clamped to
+  // the configured yAxis max of 10, rather than disappearing entirely.
+  expect(data).toHaveLength(7);
+  expect(data[4][1]).toBe(10);
+});
+
+test('clamps series values to the yAxis min when a value falls below it', () => {
+  const queriesData: ChartDataResponseResult[] = [
+    createTestQueryData(
+      createTestData(
+        [{ 'Series A': -1000 }, { 'Series A': 2 }, { 'Series A': 3 }],
+        { intervalMs: 300000000 },
+      ),
+    ),
+  ];
+
+  const chartProps = createTestChartProps({
+    formData: {
+      ...formData,
+      groupby: [],
+      seriesType: EchartsTimeseriesSeriesType.Line,
+      truncateYAxis: true,
+      yAxisBounds: [0, 10],
+    },
+    queriesData,
+  });
+
+  const transformedProps = transformProps(chartProps);
+  const series = transformedProps.echartOptions.series as SeriesOption[];
+  const seriesA = series.find(s => s.name === 'Series A');
+  expect(seriesA).toBeDefined();
+  const data = seriesA!.data as [number, number][];
+
+  expect(data).toHaveLength(3);
+  expect(data[0][1]).toBe(0);
+});
+
 test('legend is visible on tall charts when enabled by the user', () => {
   const chartProps = createTestChartProps({
     height: 400,