Unify live metrics chart rendering

bible-local/architecture/charting.md

@@ -9,6 +9,34 @@ All live metrics charts in the web UI are server-side SVG images served by Go
 and polled by the browser every 2 seconds via `<img src="...?t=now">`.
 There is no client-side canvas or JS chart library.
 
+## Rule: live charts must be visually uniform
+
+Live charts are a single UI family, not a set of one-off widgets. New charts and
+changes to existing charts must keep the same rendering model and presentation
+rules unless there is an explicit architectural decision to diverge.
+
+Default expectations:
+
+- same server-side SVG pipeline for all live metrics charts
+- same refresh behaviour and failure handling in the browser
+- same canvas size class and card layout
+- same legend placement policy across charts
+- same axis, title, and summary conventions
+- no chart-specific visual exceptions added as a quick fix
+
+Current default for live charts:
+
+- legend below the plot area when a chart has 8 series or fewer
+- legend hidden when a chart has more than 8 series
+- 10 equal Y-axis steps across the chart height
+- 1400 x 360 SVG canvas with legend
+- 1400 x 288 SVG canvas without legend
+- full-width card rendering in a single-column stack
+
+If one chart needs a different layout or legend behaviour, treat that as a
+design-level decision affecting the whole chart family, not as a local tweak to
+just one endpoint.
+
 ### Why go-analyze/charts
 
 - Pure Go, no CGO — builds cleanly inside the live-build container
@@ -29,7 +57,8 @@ self-contained SVG renderer used **only** for completed SAT run reports
 | `GET /api/metrics/chart/server.svg` | CPU temp, CPU load %, mem load %, power W, fan RPMs |
 | `GET /api/metrics/chart/gpu/{idx}.svg` | GPU temp °C, load %, mem %, power W |
 
-Charts are 1400 × 280 px SVG. The page renders them at `width: 100%` in a
+Charts are 1400 × 360 px SVG when the legend is shown, and 1400 × 288 px when
+the legend is hidden. The page renders them at `width: 100%` in a
 single-column layout so they always fill the viewport width.
 
 ### Ring buffers