PR Summary

Closes #18547

For pcolor(mesh) the new "nearest" interpolation was being done before unit conversion, so datetime (for instance) did not work. This moves unit conversion inside _pcolorargs.

from datetime import datetime

from matplotlib import pyplot as plt

x_coords = [datetime.fromtimestamp(x * 3600) for x in range(24)]
y_coords = [*range(0, 10)]
plt.pcolormesh(x_coords,
               y_coords, [[*range(24)] for _ in range(10)],
               shading="nearest")

plt.show()

failed before, and works now...

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.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and pydocstyle<4 and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).