I had something like for i in range(n): plot(..., f"oC{i}") which
works for n up to 10, but for greater values one gets "oC10" which is
not supported (which is reasonable, as some single-digit strings are
actually marker abbreviations); mentioning the full format string in
the error message ("Unrecognized character 0 in format string 'oC10'")
makes it clearer.

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• 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).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).