line-tracing

How automatic line tracing works — traceLine() in src/auto/trace/ (one click on a plotted series → its centreline or markers → suggested points) — and its benchmark (bench/lines/, npm run bench:lines): the fixed TraceRequest/TraceResult contract the Data-step UI codes against, the colour model, the column walker with gap bridging and retry, step corners, dotted lines, markers, resampling, the corpus and its truth format, and how the baseline gates regressions. Load it BEFORE editing anything in src/auto/trace/ or bench/lines/, before changing the contract or the worker/UI code that calls traceLine, when bench:lines reports a regression, when a trace follows the wrong series, loses a step corner or stops early, and when adding a figure to the line corpus. Skipping it risks breaking the UI's contract, DOM code in the worker-safe engine, "fixing" a failure by tuning the corpus, seeds or baseline instead of the tracer, or a suggestion applied without the user's Accept.