Fix stacked areas overflow on null values when connectNulls is true

src/chart/line/helper.ts

@@ -24,6 +24,7 @@ import type Cartesian2D from '../../coord/cartesian/Cartesian2D';
 import SeriesData from '../../data/SeriesData';
 import Axis from '../../coord/Axis';
 import type { LineSeriesOption } from './LineSeries';
+import { SeriesModel } from '../../echarts';
 
 interface CoordInfo {
     dataDimsForPoint: string[]
@@ -121,7 +122,12 @@ export function getStackedOnPoint(
     if (dataCoordInfo.stacked) {
         value = data.get(data.getCalculationInfo('stackedOverDimension'), idx) as number;
     }
-    if (isNaN(value)) {
+
+    const seriesModel = data.hostModel as SeriesModel;
+    const seriesValue = seriesModel.getRawValue(idx) as number;
+
+    // If there's a series value but no stacked value, start the area from the coordinate system start value
+    if (isNaN(value) && !isNaN(seriesValue)) {
         value = dataCoordInfo.valueStart;
     }