fix: re-render legend when isSecondary changes

[Pixselve]

Description

Previously, updating a series to the left axis to the right axis updated the chart, but not the legend. This was due to a missing check in the ChartExtraLegend.

How has this been tested?

2026-04-21_11-54-22.mp4

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• ✅ Changes include appropriate documentation updates.
• ✅ Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• ✅ Changes do not include unsupported browser features, see CONTRIBUTING.md.
• ✅ Changes were manually tested for accessibility, see accessibility guidelines.

Security

• ✅ If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• ✅ Changes are covered with new/existing unit tests?
• ✅ Changes are covered with new/existing integration tests?

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