`legend`

Legends: where they go, how wide they are, and the four kinds that get drawn.

Split out of plot.mojo. A legend is laid out before anything is drawn, because its width comes off the plot area – _legend_layout and _legend_reserve_for answer “how much room does this chart owe its legend” so the frame can be sized around the answer.

Four kinds: the categorical swatch column (_draw_legend, and _draw_legend_at for marks that place their own), and continuous color and size ramps, each in a vertical and a horizontal form.