Fix disconnections every 20sec and fix Y zoom

lib/ui/chart_painter.dart

@@ -79,6 +79,11 @@ class ChartPainter extends CustomPainter {
     double yForVal(double v) =>
         padTop + plotH * (1 - (v - yMin) / ySpan);
 
+    // Everything that draws into the plot area must stay within it; a
+    // Y-zoom can push line segments outside [padTop, padTop+plotH].
+    canvas.save();
+    canvas.clipRect(Rect.fromLTWH(padLeft, padTop, plotW, plotH));
+
     // Hatched gap rectangles.
     final hatchPaint = Paint()
       ..color = const Color(0x55993C1D)
@@ -134,6 +139,8 @@ class ChartPainter extends CustomPainter {
       );
     }
 
+    canvas.restore();
+
     // Axes labels.
     _drawText(canvas, '${yMax.toStringAsFixed(2)}',
         Offset(padLeft - 4, padTop), align: TextAlign.right);