Prevents axes limits from being resized by axes.fill_between

[olinjohnson]

PR Summary

Closes #25682 by preventing Axes.fill_between() from automatically adjusting the axes limits when using axes coordinates. When axes coordinates are not in use and thus no transformation is passed to Axes.fill_between(), then adjusting the axes limits is necessary, but previously, in some cases, a transformation might have been in use and using axes coordinates could have caused the axes limits to be resized improperly.

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• [N/A] New plotting related features are documented with examples.

Release Notes

• [N/A] New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• [N/A] API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• [N/A] Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst

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[ksunden]

This could probably use a test of this behavior, something like the code from #25682

I don't think it needs to be an image comparison test, just an assertion that the limits are the same before/after the fillbetween call

[QuLogic]

This should probably check the transform branches, similar to how lines do:

matplotlib/lib/matplotlib/axes/_base.py

Line 2354 in 690884f

updatex, updatey = line_trf.contains_branch_seperately(self.transData)

[ksunden]

That may be a bit more complete, yeah... for reference I based the original suggestion on axline:

matplotlib/lib/matplotlib/axes/_axes.py

Lines 910 to 913 in b86ebba

	if "transform" in kwargs:
	# if a transform is passed (i.e. line points not in data space),
	# data limits should not be adjusted.
	datalim = []

[olinjohnson]

@QuLogic I'm not yet very well-versed with transforms - wondering if you could provide some guidance as to how this would work?

I will continue to look into this as well.

[QuLogic]

See #25324 for a similar application of the transform branches for update_datalim on vlines/hlines. (Though I don't know for sure that you also need to handle rectilinear specifically as is done in that PR; just the transform bit might be enough.)

[olinjohnson]

@QuLogic @ksunden I updated the PR to check transform branches - can you review this to make sure everything looks alright?

[olinjohnson]

@ksunden I did a few basic image tests after initially making the changes:


As you can see, the example using the code from the documentation remains the same (as expected), and the example using the code for reproduction from #25712 returns the expected output after the change.

Additionally, I also tested with an assertion (per your recommendation) which verified that the axes limits remain the same before and after the fill_between call.

original_lims = (ax.get_xlim(), ax.get_ylim())

ax.fill_between(x, 0, 1, where=y > threshold2,
                color='green', alpha=0.5, transform=ax.get_xaxis_transform())

assert (ax.get_xlim(), ax.get_ylim()) == original_lims

[ksunden]

@olinjohnson could you make this a unit test in lib/matplotlib/tests/test_axes.py?

https://matplotlib.org/stable/devel/testing.html#writing-a-simple-test

This will help us to ensure that the expected behavior is retained through future changes.

[olinjohnson]

Yes! I added the unit test - sorry, should have done this before.

[ksunden]

Thank you for your contribution, hope to hear from you again!