Chart: an optional y axis title

The unit stays on the ticks; `yLabel` names the axis. It is part of the
chart's rebuild identity, so editing the title takes effect at once.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HTsT1isxUjw5gtkJk8WhuA

frontend/src/components/Common/UplotChart.tsx

@@ -76,6 +76,7 @@ export function UplotChart({
   empty = "Nothing has come through yet.",
   unit,
   yRange,
+  yLabel,
   onCursor,
   onSelect,
 }: {
@@ -88,6 +89,8 @@ export function UplotChart({
   unit?: string
   /** A y axis fixed to these bounds; unset lets it follow the data. */
   yRange?: [number, number]
+  /** A named y axis; the unit stays on the ticks. */
+  yLabel?: string
   /** The x value under the pointer, and null once it leaves the plot. */
   onCursor?: (ts: number | null) => void
   /** The x value clicked, or null for a click that landed on no point. */
@@ -107,9 +110,9 @@ export function UplotChart({
 
   const points = plots.reduce((total, plot) => total + plot.length, 0)
   // The identity of the series set: the chart is rebuilt when it changes,
-  // while a new reading only sets its data. The unit and the fixed range are
-  // part of it — both are baked into the axes when the chart is built.
-  const key = `${labels.join(" ")}|${unit ?? ""}|${yRange?.join(",") ?? ""}`
+  // while a new reading only sets its data. The unit, the fixed range and the
+  // axis title are part of it — all are baked into the axes at build time.
+  const key = `${labels.join(" ")}|${unit ?? ""}|${yRange?.join(",") ?? ""}|${yLabel ?? ""}`
   // uPlot leaves its axes half-initialised while the scales have no range, and
   // a resize in that window (a card still settling, say) draws them anyway and
   // throws. Waiting for the first reading avoids the state altogether.
@@ -170,6 +173,10 @@ export function UplotChart({
           { ...axis, size: 28 },
           {
             ...axis,
+            label: yLabel,
+            // uPlot's default label font is a hardcoded bold sans-serif, which
+            // would ignore the app's face.
+            labelFont: axis.font,
             // The unit is written after every tick, so the gutter widens to
             // hold it rather than clipping the number in front of it.
             size: unit ? 62 : 46,

frontend/src/components/Dashboard/ChartWidget.tsx

@@ -67,6 +67,7 @@ function presentation(cfg: Record<string, unknown>) {
   const high = Number(cfg.y_max)
   return {
     unit: cfg.unit ? String(cfg.unit) : undefined,
+    yLabel: cfg.y_label ? String(cfg.y_label) : undefined,
     yRange:
       Number.isFinite(low) && Number.isFinite(high) && low < high
         ? ([low, high] as [number, number])