Render Tufte Chart

This skill produces an actual chart file. It is the toolkit's output stage: the router sends "design / build / produce a chart" requests here, and an optimisation workflow ends here (assess to diagnose, render to rebuild).

The old version of this skill shipped Python that called undefined functions and could not run. It is replaced by a working script plus clear construction guidance.

Build checklist (apply every time)

These bake in the principles from references/tufte-principles.md so the chart never needs cleanup afterwards. Read that file for the full reasoning.