{% comment %}
Polar, so it carries its own measurement rather than going inside the plot, and
the arc maths lives here because nothing cartesian needs it.
Each slice is its own overlaid drawing surface holding a single path. One
surface with a path per slice would need a repeat inside SVG, which renders
exactly one slice; this way the repeat stays in HTML, and each slice keeps a
hit area of its own for the tooltip.
{% endcomment %}