Fix failing figure association for non-faceting plots

The error occurred if a non-faceting plot created a new figure, which is the case if arguments like size or aspect are passed to xarray's facet grid function.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved by @tgaskin