[snario]

Builds on #18 and includes its commit. The new commit is the last one.

Some charts put two quantities with different units or magnitudes on one chart, such as revenue and margin. On one value axis, the smaller series flattens near zero.

This adds a rightAxis option under xyChart. Series named in rightAxis.series are measured against a second value axis:

• Placement: the axis is drawn on the right of the plot, or along the top when the chart runs horizontal.
• Range: min and max set the range. Without them, the axis fits its series, and bars stand on zero.
• Title: title sets the axis title. showTitle, showLabel, showTick, and showAxisLine work as on the other axes.
• Stacking: with stacked: true, bars of each axis form their own stack, and the two stacks stand side by side.

The option is configuration only, so the parser and diagram model stay identical to Mermaid 12.1.0, which ignores the key.

---
config:
  xyChart:
    rightAxis:
      series: [Margin]
      title: "Margin (%)"
      min: 0
      max: 50
---
xychart
  x-axis [Q1, Q2, Q3, Q4]
  y-axis "Revenue ($M)"
  bar "Revenue" [120, 180, 240, 310]
  line "Margin" [12, 18, 22, 29]

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Validation: cargo clippy, cargo test --workspace, pnpm check, pnpm check:types, vp test --project node (806 tests) and pnpm check:size pass. Every existing XY chart vector renders byte-identical SVG. The browser screenshot suite was not run.