Infer dodge grouping from fill in position_jitterdodge()

[Jesssullivan]

Infer dodge grouping from fill in position_jitterdodge()

Closes #6824.

Motivation

position_jitterdodge() is documented as "primarily used for aligning points generated through geom_point() with dodged boxplots (e.g., a geom_boxplot() with a fill aesthetic supplied)." However, the function currently derives dodge grouping from the full implicit group interaction, which includes every discrete aesthetic in the layer — not just fill. Adding colour, shape, or linetype silently breaks the alignment this function is designed to provide.

# Points misalign because group = cyl x am x vs, but boxplot dodges by cyl x am
ggplot(mtcars, aes(factor(cyl), mpg, fill = factor(am))) +
  geom_boxplot(outlier.shape = NA) +
  geom_point(aes(colour = factor(vs)),
             position = position_jitterdodge(seed = 1))

Approach

Since this function exists specifically for boxplot+point alignment, and boxplots dodge by fill, the dodge grouping should derive from fill when available.

A 6-line unexported helper (jitterdodge_dodge_group) checks whether fill is present and discrete, returning either a fill-based group or falling back to the standard group. Three methods (setup_params, setup_data, compute_panel) use this helper to temporarily swap the group for dodging, then restore the original group for downstream use. No new parameters are added.

Note on existing behaviour

We understand that ggplot2 is a mature package and that PRs changing existing behaviour require careful consideration. This change only affects the case where the current output is visually incorrect:

Scenario	Before	After	Change?
fill only	Dodge by fill	Dodge by fill	Identical
fill + colour	Dodge by fill x colour	Dodge by fill	Fix
No fill	Dodge by group	Dodge by group	Identical
Continuous fill	Dodge by group	Dodge by group	Identical

The only behaviour change is in the case where the current result contradicts the function's documented purpose. Users who need the old grouping can use aes(group = interaction(fill, colour)) explicitly.

Changes

• R/position-jitterdodge.R: jitterdodge_dodge_group() helper + modifications to setup_params, setup_data, compute_panel
• R/position-jitterdodge.R: New @section Dodge grouping: documenting the inference and fallback
• tests/testthat/test-position-jitterdodge.R: 5 new tests (alignment, backward compat, no-fill fallback, continuous-fill fallback, existing preserve tests still pass)
• NEWS.md: Entry added

Checklist

• Motivation described above and in NEWS entry (@Jesssullivan, #6824)
• Only related changes (single concern: fill-based dodge inference)
• Tidyverse style — follows existing patterns, reuses id(), is_discrete()
• roxygen2 docs updated, devtools::document() run
• Unit tests added (5 new, all passing; 22 position-dodge regression tests also pass)
• Visual tests — N/A
• Minimal example added to @examples

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[Jesssullivan]

FWIW, I prefer this inference solution to retroactive documentation route Draft PR - 6825, keen to hear others thoughts @krosepulham 👀