fix(timeseries): cap the percent-change baseline to the plot area

The handle spanned the full chart height, running through the legend and
x-axis labels; it now uses the grid's actual rect, falling back to full
height if the internal API is unavailable.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: claude

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

@@ -127,7 +127,31 @@ export default function EchartsTimeseries({
     };
 
     const drawHandle = () => {
-      const gridRect = { top: 0, height: chart.getHeight() };
+      // Cap the handle to the plot area so it doesn't run through the
+      // legend above or the axis labels below.
+      let gridRect = { top: 0, height: chart.getHeight() };
+      try {
+        const rect = (
+          chart as unknown as {
+            getModel: () => {
+              getComponent: (
+                type: string,
+                index: number,
+              ) => {
+                coordinateSystem: {
+                  getRect: () => { y: number; height: number };
+                };
+              };
+            };
+          }
+        )
+          .getModel()
+          .getComponent('grid', 0)
+          .coordinateSystem.getRect();
+        gridRect = { top: rect.y, height: rect.height };
+      } catch {
+        // fall back to the full chart height
+      }
       let px: number;
       try {
         [px] = [chart.convertToPixel({ xAxisIndex: 0 }, baselineX) as number];