PR Summary

Deal withimshowing an array that is completely masked without emitting a warning. The added test/code emits 3 warnings onmaster, none of which are particularly informative. The test onmaster fails with:

lib/matplotlib/image.py:639: in draw    im, l, b, trans = self.make_image(lib/matplotlib/image.py:924: in make_image    return self._make_image(self._A, bbox, transformed_bbox, clip,lib/matplotlib/image.py:476: in _make_image    vrange = np.array([vmin, vmax], dtype=scaled_dtype)_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ self = masked    def __float__(self):        """        Convert to float.            """        if self.size > 1:            raise TypeError("Only length-1 arrays can be converted "                            "to Python scalars")        elif self._mask:>           warnings.warn("Warning: converting a masked element to nan.", stacklevel=2)E           UserWarning: Warning: converting a masked element to nan.

And on latestNumPy it also emits a warning later having to do withnp.clip not likingnp.nan values.

Alternatively,imshow with all data masked could emit a warning, but it seems like the existing code had a chance to do that but didn't (there is an existingif a_min is np.ma.masked on line 409) so I didn't implement it here.

PR Checklist

• Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (runflake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• Conforms to Matplotlib style conventions (installflake8-docstrings andpydocstyle<4 and runflake8 --docstring-convention=all).
• New features have an entry indoc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented indoc/api/next_api_changes/ (follow instructions in README.rst there).