PR Summary

This fixes the issue described in #14274 that plotting with Astropy Quantities is now very slow in Matplotlib 3.1.0.

Note that given the existing test, the code could have even been written as:

    if len(X) == 0:
        return [[]]
    elif X.ndim == 1:
        # 1D array of scalars: directly return it.
        return [X]

so I've also added a more involved test to make sure that what #11921 was trying to fix still works, as well as a regression test for #14274

cc @anntzer and @jklymak

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