PR Summary

ScalarFormatter.format_data_short uses self.axis.__name__ to determine whether it is working with an x-axis, a y-axis, or something else, presumably a _DummyAxis instance. This was failing in the z-cursor formatting of image-like plots for a the colorbar use case with boundaries specified. This could be solved either by checking first for the presence of the __name__ attribute or by giving _DummyAxis instances a name. I chose the latter approach.

Closes #17624

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