PR Summary

d = {4:1, 6:2}
plt.plot(d.keys(), d.values())

is supposed to work & then stopped working because of changes to how .plot processes the data kwarg (#10928). This PR takes the dead simple approach of running the args to .plot through sanitize_sequence since it's a pass through function, but there's probably a better way.

Also added a bunch of tests to hopefully catch this. I think this is release critical since it worked in 3.0 and broke in 3.1, but on the other hand I only noticed this 'cause I wanted to update some docs (that actually should have been fine even w/o the fix 🙃)

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way