Sided twin violins render boxplot whiskers incorrectly

[rabyj]

Plotly version: 6.3.0

Follow-up to #1634 (never resolved).

When using sided (split) violins with box=dict(visible=True), the boxplot whiskers are drawn at the inner edge of each half-box, rather than being centered as in a normal boxplot.

This makes the left and right whiskers appear extremely close together, which creates significant visual confusion.

Example from #1634 (Advanced Violin Plot example with box=dict(visible=True)):

My reproduction:

Normal (non-sided) violins for comparison:

Plotly 6.1.2 (in my work):

Possible cause:
The boxplots may be internally split in half to fit each half-violin, shifting the “center” used for whisker rendering to the midpoint between the halves.

Possible solutions:

• Preferred: Render each half-boxplot with whiskers centered over its own half-violin.
• Alternative: Allow configurable spacing between the halves to reduce whisker overlap.