What a built plot draws, as numbers: per layer (and facet panel) and per
series, how many marks, the x range, the y minimum / maximum and where,
the first and last y, and a band’s extent. A caller that writes about a
chart (a title, an answer) can quote these instead of guessing them.
Convert a polars DataFrame into plot-ready columns using your
crate’s polars (#466): the macro expands in your crate, against the
polars version you depend on, so it works whether you use polars 0.46,
0.52, 0.53 or later — no second polars build, no type mismatch, and
ggplot-rs’s own polars feature can stay off.