Reject scalar coordinates as plot axes

[sdraeger]

Description

Closes #7333.

Facet coordinates remain attached as zero-dimensional coordinates after FacetGrid selects each panel. _infer_xy_labels accepted those coordinates as plotting axes, causing the 2D plotting code to index a nonexistent first dimension and raise IndexError.

This change rejects zero-dimensional x/y coordinates during label inference and raises a clear ValueError instead. It also adds a regression test covering a faceting coordinate passed as an axis.

Tests

• uv run pytest xarray/tests/test_plot.py
• pre-commit run --files xarray/plot/utils.py xarray/tests/test_plot.py

Checklist

• Closes FacetGrid with coords error #7333
• Tests added
• User visible changes are documented in whats-new.rst

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Codex

[welcome]

Thank you for opening this pull request! It may take us a few days to respond here, so thank you for being patient.
If you have questions, some answers may be found in our contributing guidelines.