This is admittedly more of a band-aid than a proper fix, but at least
make the generated figure correct.

The underlying problem is the more fundemental one of inconsistent units
behavior in which some artists can be re-united after plotting (e.g.
Line2D), while many others (e.g. Collections) cannot.

Thus, until that is addressed (which is one of the goals of the
data-prototype work), simply setting units early is as good as we can
do.

Closes#5051

PR Summary

PR Checklist

Documentation and Tests

• Has pytest style unit tests (andpytest passes)
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a.. versionadded:: directive in the docstring and documented indoc/users/next_whats_new/
• API changes are marked with a.. versionchanged:: directive in the docstring and documented indoc/api/next_api_changes/
• Release notes conform with instructions innext_whats_new/README.rst ornext_api_changes/README.rst