[thclark]

This reworks stages 1 and 3 of the bars and histograms epic, following the design review on 2026-10-26.

Bars as a primitive

• A bar is drawn at its position plus its series' offset. The offset is a multiple of the bar's width, or a distance along the category axis.
• Its width is a fraction of the pitch, a constant, or one width per bar. It runs from its base to its base plus its value, so each value is a length, as in Matplotlib and Plotly.
• A margin (default 0.05) extends the automatic value limits beyond the bar ends, so the tallest bar no longer touches the box.
• Bar groups, slots and gaps are removed from the model, along with the InconsistentBarGroup warning.

The bars helper

The facade's bars helper writes the offsets, widths and bases of grouped columns, stacked columns, and stacks side by side.

Stored bin definitions

• Bars, stairs and cells gain an optional record of how a histogram was binned: the bins as given, the range, the normalisation, the total and the exclusions.
• Datatips use the record to report each bin's count and density.
• Bins, BinRule and HistNorm become model types.

ADRs

The decisions are recorded in four ADRs:

• 0026: bars are a primitive, coloured as series are.
• 0027: cells are an artist of their own.
• 0028: closed shapes have faces and edges, and lines have fills.
• 0029: histograms store their bins and how they were made.

Existing gallery figures render byte-identically.