PR summary

Closes #19021.

Axis.get_gridlines() currently returns only major gridlines, with no public way to obtain the minor gridlines. As called out in the issue thread, downstream libraries have been forced to reach into private API as a workaround:

• seaborn reads Axis._minor_tick_kw['gridOn'] (source)
• gwpy, mpld3, mplexporter and others were referenced in the same thread

This PR implements the API @jklymak proposed in the issue:

Or a new get_gridlines(which='minor'/'major'/'both')?

def get_gridlines(self, which='major'):
    """Return this Axis' grid lines as a list of Line2D's."""

The vocabulary 'major' | 'minor' | 'both' matches Axis.grid() and Axis.get_ticklabels(). Default 'major' preserves the previous behavior byte-for-byte, so every existing zero-argument caller continues to work unchanged.

Files changed

Testing

I monkey-patched the new implementation onto an installed matplotlib==3.10.9 and ran the same assertions the new pytest cases exercise:

• selection by which='major', which='minor', which='both' returns the expected lengths and ordering
• the 'both' ordering is major + minor (matching get_ticklabels)
• visibility on the returned Line2D objects tracks the active grid state through Axis.grid(visible=..., which=...) calls
• backward compatibility: get_gridlines() and get_gridlines('major') are equivalent on both xaxis and yaxis
• which='invalid' raises ValueError with the standard _api.check_in_list message
• Axes.get_xgridlines(which=...) / Axes.get_ygridlines(which=...) auto-delegate via _axis_method_wrapper

All checks pass against the installed matplotlib with the new method patched in.

Notes for reviewers

• The implementation follows @jklymak's exact API suggestion. If you would prefer separate get_majorgridlines / get_minorgridlines methods (briefly considered in the thread) I am happy to refactor.
• For which='both', major gridlines come before minor gridlines, matching get_ticklabels(which='both').
• The docstring's Notes section explicitly tells callers to check Line2D.get_visible() to address @anntzer's concern about per-gridline visibility.

AI Disclosure

AI was used to help proofread the grammar of this PR description and to understand some parts of the existing codebase during development. The code changes and design decisions were developed independently.

PR checklist

• "closes Need a way to check if an axis minor or major grid is on #19021" is in the body of the PR description
• new and changed code is tested
• Plotting related features are demonstrated in an example (the What's new entry contains a .. plot:: directive showing minor-gridline coloring)
• New Features and API Changes are noted with a directive and release note (versionchanged:: 3.11 in the docstring + doc/release/next_whats_new/get_gridlines_which.rst)
• Documentation complies with general and docstring guidelines