[snario]

Multiple bar series in an XY chart render side by side. Parts of a whole, such as orders by channel, read better as one bar per category whose segments add up to the total.

This adds a stacked option under xyChart. With stacked: true, all bar series of a chart draw as one bar per category:

• Direction: positive values stack above the baseline, and negative values stack below it.
• Segments: a 1 px line of the background separates each segment from the one under it.
• Range: without a y-axis range, the value axis fits the stack totals.
• Labels: with showDataLabel, each stack shows its total, or both totals when it goes both ways.
• Lines: line series draw over the stacks, unchanged.

The option is configuration only, so the parser and diagram model stay identical to Mermaid 12.1.0, which ignores the key. mermaid-js/mermaid#7509 proposes a brace syntax for stacking, but it is not merged, so this PR adds no syntax.

---
config:
  xyChart:
    stacked: true
    showDataLabel: true
---
xychart
  x-axis [Q1, Q2, Q3, Q4]
  bar "Online" [12, 15, 18, 22]
  bar "Store" [8, 7, 9, 10]
  bar "Partner" [3, 4, 6, 7]

Loading

The XY chart page documents the option, and a changeset marks it as a minor release.

Validation: cargo clippy, cargo test --workspace, pnpm check, pnpm check:types, vp test --project node (805 tests) and pnpm check:size pass. Every existing XY chart vector renders byte-identical SVG. The browser screenshot suite was not run.