Redesign system power chart as stacked per-PSU area chart

- Add PSUReading struct and PSUs []PSUReading to LiveMetricSample
- Sample per-PSU input watts from IPMI SDR entity 10.x (Power Supply)
- Render stacked filled-area SVG chart (one layer per PSU, cumulative total)
- Fall back to single-line chart on systems with ≤1 PSU in SDR

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/internal/webui/charts_svg.go

@@ -462,6 +462,127 @@ func synthesizeChartTimes(times []time.Time, count int) []time.Time {
 	return out
 }
 
+// renderStackedMetricChartSVG renders a stacked area chart where each dataset
+// is visually "stacked" on top of the previous one. Intended for multi-PSU
+// power charts where the filled area of each PSU shows its individual
+// contribution and the total height equals the combined draw.
+func renderStackedMetricChartSVG(title string, labels []string, times []time.Time, datasets [][]float64, names []string, yMax *float64, canvasHeight int, timeline []chartTimelineSegment) ([]byte, error) {
+	pointCount := len(labels)
+	if len(times) > pointCount {
+		pointCount = len(times)
+	}
+	if pointCount == 0 {
+		pointCount = 1
+		labels = []string{""}
+		times = []time.Time{{}}
+	}
+	if len(labels) < pointCount {
+		padded := make([]string, pointCount)
+		copy(padded, labels)
+		labels = padded
+	}
+	if len(times) < pointCount {
+		times = synthesizeChartTimes(times, pointCount)
+	}
+	for i := range datasets {
+		if len(datasets[i]) == 0 {
+			datasets[i] = make([]float64, pointCount)
+		}
+	}
+
+	times, datasets = downsampleTimeSeries(times, datasets, 1400)
+	pointCount = len(times)
+
+	// Build cumulative sums per time point.
+	cumulative := make([][]float64, len(datasets)+1)
+	for i := range cumulative {
+		cumulative[i] = make([]float64, pointCount)
+	}
+	for i, ds := range datasets {
+		for j, v := range ds {
+			cumulative[i+1][j] = cumulative[i][j] + v
+		}
+	}
+
+	// Scale is based on the total (top cumulative row).
+	total := cumulative[len(cumulative)-1]
+	yMin := floatPtr(0)
+	if yMax == nil {
+		yMax = autoMax120(total)
+	}
+	scale := singleAxisChartScale([][]float64{total}, yMin, yMax)
+
+	legendItems := make([]metricChartSeries, len(datasets))
+	for i, name := range names {
+		color := metricChartPalette[i%len(metricChartPalette)]
+		legendItems[i] = metricChartSeries{Name: name, Color: color, Values: datasets[i]}
+	}
+
+	// Stats label from totals.
+	statsLabel := chartStatsLabel([][]float64{total})
+
+	layout := singleAxisChartLayout(canvasHeight, len(legendItems))
+	start, end := chartTimeBounds(times)
+
+	var b strings.Builder
+	writeSVGOpen(&b, layout.Width, layout.Height)
+	writeChartFrame(&b, title, statsLabel, layout.Width, layout.Height)
+	writeTimelineIdleSpans(&b, layout, start, end, timeline)
+	writeVerticalGrid(&b, layout, times, pointCount, 8)
+	writeHorizontalGrid(&b, layout, scale)
+	writeTimelineBoundaries(&b, layout, start, end, timeline)
+	writePlotBorder(&b, layout)
+	writeSingleAxisY(&b, layout, scale)
+	writeXAxisLabels(&b, layout, times, labels, start, end, 8)
+
+	// Draw stacked areas from top to bottom so lower layers are visible.
+	for i := len(datasets) - 1; i >= 0; i-- {
+		writeStackedArea(&b, layout, times, start, end, cumulative[i], cumulative[i+1], scale, legendItems[i].Color)
+	}
+	// Draw border polylines on top.
+	for i := len(datasets) - 1; i >= 0; i-- {
+		writeSeriesPolyline(&b, layout, times, start, end, cumulative[i+1], scale, legendItems[i].Color)
+	}
+
+	writeLegend(&b, layout, legendItems)
+	writeSVGClose(&b)
+	return []byte(b.String()), nil
+}
+
+// writeStackedArea draws a filled polygon between two cumulative value arrays
+// (baseline and top), using the given color at 55% opacity.
+func writeStackedArea(b *strings.Builder, layout chartLayout, times []time.Time, start, end time.Time, baseline, top []float64, scale chartScale, color string) {
+	n := len(top)
+	if n == 0 {
+		return
+	}
+	if len(baseline) < n {
+		baseline = make([]float64, n)
+	}
+
+	// Forward path along top values, then backward along baseline values.
+	var points strings.Builder
+	for i := 0; i < n; i++ {
+		x := chartXForTime(chartPointTime(times, i), start, end, layout.PlotLeft, layout.PlotRight)
+		y := chartYForValue(valueClamp(top[i], scale), scale, layout.PlotTop, layout.PlotBottom)
+		if i > 0 {
+			points.WriteByte(' ')
+		}
+		points.WriteString(strconv.FormatFloat(x, 'f', 1, 64))
+		points.WriteByte(',')
+		points.WriteString(strconv.FormatFloat(y, 'f', 1, 64))
+	}
+	for i := n - 1; i >= 0; i-- {
+		x := chartXForTime(chartPointTime(times, i), start, end, layout.PlotLeft, layout.PlotRight)
+		y := chartYForValue(valueClamp(baseline[i], scale), scale, layout.PlotTop, layout.PlotBottom)
+		points.WriteByte(' ')
+		points.WriteString(strconv.FormatFloat(x, 'f', 1, 64))
+		points.WriteByte(',')
+		points.WriteString(strconv.FormatFloat(y, 'f', 1, 64))
+	}
+	fmt.Fprintf(b, `<polygon points="%s" fill="%s" fill-opacity="0.55" stroke="none"/>`+"\n", points.String(), color)
+}
+
 func writeSVGOpen(b *strings.Builder, width, height int) {
 	fmt.Fprintf(b, `<svg xmlns="http://www.w3.org/2000/svg" width="%d" height="%d" viewBox="0 0 %d %d">`+"\n", width, height, width, height)
 }