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Fixed a bug with drawing an empty Collection - #32228

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ayshih wants to merge 3 commits into
matplotlib:mainfrom
ayshih:empty_scatter
Open

Fixed a bug with drawing an empty Collection#32228
ayshih wants to merge 3 commits into
matplotlib:mainfrom
ayshih:empty_scatter

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@ayshih

@ayshih ayshih commented Aug 18, 2026 •
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PR summary

The PR fixes a bug (fixes #32219) with drawing an empty scatter plot creating a PathCollection with an empty offsets list. Renderers can interpret this (incorrectly) as wanting a single marker with no offset rather than no markers. This PR simply bails out of the draw() call when offsets is empty.

Fixes a bug revealed in #32219, where specifying facecolor="none" (or edgecolor="none") went down a different code path than specifying a color. It turns out that specifying "none" would not be understood as a single color – because zero colors is not equal to one color – which would prevent the optimized drawing of a collection when there is just a single path (allowing the use of draw_markers() instead of draw_path_collection()). I'm going to create an issue for this because it opened up a can of worms. Edit: Ah, #17790 already exists, so I have created #32232 for discussion.

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PR quality check

  • Use an expressive title, e.g. "Fix title font property precedence"
  • New and changed code is tested
  • [N/A] Plotting related features are demonstrated in an example
  • [N/A] New features and API changes have release notes
  • [N/A] Documentation complies with general and docstring guidelines

@ayshih ayshih changed the title Fixed a bug with drawing an empty Collection and a second bug with single-path optimization Fixed a bug with drawing an empty Collection Aug 19, 2026
@ayshih ayshih mentioned this pull request Aug 19, 2026
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ayshih marked this pull request as ready for review August 19, 2026 13:45
@QuLogic

QuLogic commented Aug 19, 2026

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This makes for a subtle difference; empty scatters won't do any of the bookkeeping that's done for non-empty ones (or even other empty Collection). The main thing there being the open_group/close_group call (which really only affects SVG, I think).

I'm not sure we want to introduce that inconsistency over clarifying backend behaviour.

@ayshih

ayshih commented Aug 19, 2026

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For that concern, I could have draw() still call open_group()/close_group() instead of completely bailing out right at the beginning. That said, if get_visible() == False already makes draw() bail out before that bookkeeping, does that bookkeeping actually matter for something that is not supposed to actually draw anything?

@tacaswell

tacaswell commented Aug 20, 2026

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"Artist marked as not visible" and "artist with null data" are different.

It is worth it's own if block that just does open/close.

@ayshih

ayshih commented Aug 20, 2026

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Okay, just making sure. Now updated.

QuLogic
QuLogic reviewed Aug 20, 2026
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Comment thread lib/matplotlib/collections.py
@ayshih @QuLogic
Update lib/matplotlib/collections.py
886fc82 
Co-authored-by: Elliott Sales de Andrade <quantum.analyst@gmail.com>
@QuLogic QuLogic added this to the v3.11.2 milestone Aug 20, 2026
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topic: collections and mappables

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v3.11.2

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[Bug]: ax.scatter([], [], clip_on=False, label=...) renders a ghost marker at the origin/corner when combined with specific styles/limits

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