Live chart lets a held window go once its points have scrolled off

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

frontend/src/components/Common/UplotChart.tsx

@@ -466,9 +466,17 @@ export function UplotChart({
   // is held the data goes in and the scales stay where they were put. Clearing
   // the window renders, which brings the next pass through here with the reset
   // back on: that is what puts the whole range back.
+  //
+  // A live chart keeps a count of points rather than a span, so a held window
+  // scrolls off the left as readings arrive. Once its last point has gone it
+  // is let go, or the chart would sit empty until somebody reset it.
   useEffect(() => {
-    if (!chart.current || plots.length === 0) return
-    chart.current.setData(table(plots), !zoomed.current)
+    const plot = chart.current
+    if (!plot || plots.length === 0) return
+    const data = table(plots)
+    const held = plot.scales.x.max
+    if (zoomed.current && held != null && held < data[0][0]) clearZoom()
+    plot.setData(data, !zoomed.current)
   })
 
   // The canvas cannot follow a CSS variable, so a theme swap is a redraw. The