fix: Detect touches on downward (negative) BarChartRodStackItems

lib/src/chart/bar_chart/bar_chart_painter.dart

@@ -943,7 +943,12 @@ class BarChartPainter extends AxisChartPainter<BarChartData> {
             final stackItem = nearestBarRod.rodStackItems[stackIndex];
             final fromPixel = getPixelY(stackItem.fromY, viewSize, holder);
             final toPixel = getPixelY(stackItem.toY, viewSize, holder);
-            if (touchedPoint.dy <= fromPixel && touchedPoint.dy >= toPixel) {
+            // A negative-direction stack item has fromY > toY, in which case
+            // fromPixel < toPixel (Flutter Y grows downward). Compare against
+            // the [min, max] range so the check works in either orientation.
+            final topPixel = min(fromPixel, toPixel);
+            final bottomPixel = max(fromPixel, toPixel);
+            if (touchedPoint.dy >= topPixel && touchedPoint.dy <= bottomPixel) {
               touchedStackIndex = stackIndex;
               touchedStack = stackItem;
               break;

test/chart/bar_chart/bar_chart_painter_test.dart

@@ -3398,6 +3398,48 @@ void main() {
         expect(result1.touchedRodDataIndex, 0);
       },
     );
+
+    test('detects touches on downward (negative) stack items', () {
+      const viewSize = Size(200, 100);
+      final barGroups = [
+        BarChartGroupData(
+          x: 0,
+          barRods: [
+            BarChartRodData(
+              fromY: 0,
+              toY: -10,
+              width: 20,
+              color: const Color(0xFF0000FF),
+              rodStackItems: [
+                BarChartRodStackItem(0, -10, const Color(0xFFFF0000)),
+              ],
+            ),
+          ],
+        ),
+      ];
+      final (minY, maxY) = BarChartHelper().calculateMaxAxisValues(barGroups);
+      final data = BarChartData(
+        barGroups: barGroups,
+        titlesData: const FlTitlesData(show: false),
+        alignment: BarChartAlignment.center,
+        barTouchData: const BarTouchData(handleBuiltInTouches: true),
+        minY: minY,
+        maxY: maxY,
+      );
+
+      final painter = BarChartPainter();
+      final holder =
+          PaintHolder<BarChartData>(data, data, TextScaler.noScaling);
+
+      // The single downward rod (fromY: 0, toY: -10) spans the full viewport
+      // height in pixels. A touch in the middle of the chart should land on
+      // the rod's only stack item.
+      final result =
+          painter.handleTouch(const Offset(100, 50), viewSize, holder);
+      expect(result, isNotNull);
+      expect(result!.touchedStackItemIndex, 0);
+      expect(result.touchedStackItem, isNotNull);
+    });
   });
 
   group('drawExtraLines()', () {