[thclark]

Stage 6 of the bars and histograms epic.

New gallery figures

Figure	What it shows
bar_chart	Grouped bars made with bars.
bar_variants	Stacked bars with negative parts, horizontal bars with long names, floating bars, and per-bar colours.
stairs	Sample-and-hold steps, and an octave-band spectrum.
histogram	Density step outlines with fitted normals, and stacked counts.
histogram_2d	A 2D histogram on a log colour scale, beside filled density contours.
branch_coverage	The #74 branch map: grouped rows under bold headers, ragged rows and a hidden x axis.
coverage_matrix	The #74 case matrix: turned monospace names, a discrete colourbar, a not-entered key and a dot overlay.

Tests and documentation

• The gallery coverage test now requires ten new chart types.
• The registry test now refuses layout warnings.
• The guides and references link to the new figures, and the documentation touched by the epic has been checked for consistency.

Unchanged

Existing gallery outputs are byte-identical, and no library behaviour changes.