A host that re-anchors its x labels on a grid has to name the anchor somewhere, and
an `Annotation` cannot serve: its anchor is a data coordinate, so the caption crawls
across the plot under a pan and drifts under a zoom.
`Plot::set_x_note()` takes the host's own string -- the host is the only one that
knows how much precision the sum of a label and its caption needs -- and
`Renderer::draw_x_note()` places it in the axes' bottom-left corner in pixels, on the
legend's own backplate. It is measured in the renderer rather than in `Plot::layout()`
because it reserves no space and nothing can be picked on it, and a caption explaining
the x labels must not be able to move the layout those labels were measured into.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
