PR Summary

#18647 broke loglog scale axlines (see #19445). This PR fixes it by adding transScale to the transformations applied to the points of the line and adds a test to verify the result (based on the example given by @anntzer in #19445).

This should have no effect on results with linear scales, and the remaining tests are still passing, so it looks like it works correctly.

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related. [N/A]
• Documentation is sphinx and numpydoc compliant (the docs should build without error). [N/A]
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all). [N/A]
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there). [N/A]
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there). [N/A]