Keep the theme redraw from wiping a chart's x scale

Reaching Home from another page builds its charts in the same commit their
theme effect first fires in, with the readings already cached. uPlot ranges its
scales in a microtask, so `redraw()` re-set the x scale from the chart's own —
still empty — bounds before that ran, and the pending range taken from the data
was lost: axes without ticks, no lines, and no way back but the range control,
which rebuilds the chart. Redrawing without the paths is all a colour swap
needs and leaves the scales alone.

The cards also carried "Nothing has run yet." while the first readings were
still on their way; they now carry the skeleton the rest of Home uses.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SKL7sUgNWhukDEz95vSMQv

frontend/src/components/Common/UplotChart.tsx

@@ -4,6 +4,7 @@ import "uplot/dist/uPlot.min.css"
 
 import type { HistoryPoint } from "@/client"
 import { useTheme } from "@/components/theme-provider"
+import { Skeleton } from "@/components/ui/skeleton"
 import { si } from "@/lib/utils"
 
 /**
@@ -74,6 +75,7 @@ export function UplotChart({
   labels,
   plots,
   empty = "Nothing has come through yet.",
+  pending = false,
   unit,
   yRange,
   yLabel,
@@ -85,6 +87,9 @@ export function UplotChart({
   /** The points of each series, in the same order as `labels`. */
   plots: HistoryPoint[][]
   empty?: string
+  /** No readings yet because none have arrived: a chart with nothing in it
+   *  says "nothing has run", which is a different thing to say. */
+  pending?: boolean
   /** Written after every reading, on the axis and in the legend. */
   unit?: string
   /** A y axis fixed to these bounds; unset lets it follow the data. */
@@ -240,10 +245,16 @@ export function UplotChart({
     chart.current.setData(table(plots))
   }, [points, key])
 
-  // The canvas cannot follow a CSS variable, so a theme swap is a redraw.
+  // The canvas cannot follow a CSS variable, so a theme swap is a redraw. The
+  // paths are geometry and stay as they are — and leaving them alone is what
+  // keeps this safe on the mount it also fires on: a full redraw re-sets the x
+  // scale from the chart's own bounds, which are still empty when the chart was
+  // built in this same commit (uPlot ranges its scales in a microtask). That
+  // overwrites the range it was about to take from the data, and the chart is
+  // left with no x axis and no lines until something sets its data again.
   // biome-ignore lint/correctness/useExhaustiveDependencies: the theme is the signal, not something the effect reads.
   useEffect(() => {
-    chart.current?.redraw()
+    chart.current?.redraw(false)
   }, [resolvedTheme])
 
   return (
@@ -251,9 +262,13 @@ export function UplotChart({
       <div className="relative min-h-0 flex-1">
         <div ref={host} className="absolute inset-0" />
         {points === 0 ? (
-          <p className="absolute inset-0 flex items-center justify-center text-sm text-muted-foreground">
-            {empty}
-          </p>
+          pending ? (
+            <Skeleton className="absolute inset-0" />
+          ) : (
+            <p className="absolute inset-0 flex items-center justify-center text-sm text-muted-foreground">
+              {empty}
+            </p>
+          )
         ) : null}
       </div>
       {/* Kept at the legend's resting height, so the plot does not resize