PR Summary

Line2D objects are "sub sliced" if they are well-ordered, and their view window is smaller than the actual data extents. However, if someone applies a transform, in particular a lateral translation, then the data all ends up out of view. This adds a check to only apply the subslice algorithm if the transform isax.transData.

Closes#21008

PR Checklist

• Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (runflake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• Conforms to Matplotlib style conventions (installflake8-docstrings and runflake8 --docstring-convention=all).
• New features have an entry indoc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented indoc/api/next_api_changes/ (follow instructions in README.rst there).