Focus the pyplot animation demo to just 1) do some simple plotting, 2)
call pause().

Yes, it would be more efficient to reuse the same image. But if we are
not using the animation module, efficiency is probably the least of our
worries.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is PEP 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