[thclark]

Stage 4 of the bars and histograms epic. It delivers the occupancy matrices of #74, except for the margin table of counts, which is left to the table chart type (#121).

Cells

• Cells are drawn one rectangle each between their edges, unit or explicit, increasing or decreasing. The gap is measured on the page, so it stays symmetric on logarithmic and reversed axes.
• Colours come through the cells' colour scale, with the scale's out-of-range policies. NaN padding draws nothing, and a strict policy leaves the whole artist out, as for a scalar image.
• The alpha array and the face and edge opacities multiply the colour, and an automatic or colormapped edge takes its cell's colour.
• Limits are tight at the outer edges, and the legend sample is the scale's sweep, as for a scalar image.
• A 100 × 100 grid compiles in a time comparable to a surface of as many faces.
• New Tip::Cell datatips report the row and column, the value (category, index or value, as for pixels) and the row and column category names. They appear on the native canvas and on the web.

Facade

• New calls cells and occupancy. occupancy pads ragged rows with NaN, uses gaps of 0.2 and hides the x axis.
• New category helpers xcategories, ycategories (first category at the top) and ycategory_groups. The last returns a CategoryRows that places data in grouped rows under \textbf headers.

Supporting changes

• \textbf{…} is set in the bundled bold face.
• Rectangle insets and category-name lookups are shared with bars.
• The getting-started guide, the colour guide, the viewer guide, the equivalent-functions reference, the architecture reference and the figure schema are updated.

Existing gallery figures render byte-identically.