Chart recorder

A strip-chart recorder: gridded paper advancing under a pen that rides a rail, drawing the live value as the paper moves and printing the time in the margin. For p95 latency, tokens per second, grid frequency, a cold room’s log.

Readings arrive once a second. Ship the canary to push p95 over its 300 ms set-point and light the lamp; stop the chart and the pens lift.

• The recorder is a figure named by its printed label. The paper is one role="img" whose name states the data — “p95 latency, last 2 minutes: p95 now 182 ms; lowest 121 ms, highest 342 ms; p50 now 96 ms. Under 300 ms.” — rewritten at most every two seconds. A name that changes is not announced, so a live chart never floods a screen reader.
• The latest reading is text on the display, there to be read whenever it is wanted. Crossing a set-point is the one thing announced, once, politely: “p95 latency: above 300 ms — 342 ms”, and “back under 300 ms” when it recovers.
• The alarm lamp and the drive lamp always have words beside them. The second pen differs from the first by its dash as well as its tone.
• It is a display, not a control: it takes no focus. Put the keys that act on it (stop, export) beside it.
• Reduced motion: the paper steps one whole division at a time and the pens jump to their readings; every reading is still drawn.