arctest.py was (from the old changelog entries) intended as a test for
the long-obsoleted draw_arc method; it's not a particularly relevant
example otherwise (there are better examples for showcasing markers).
It's also right now at the very top of the examples list, which it
doesn't deserve.
https://matplotlib.org/gallery/lines_bars_and_markers/arctest.html#sphx-glr-gallery-lines-bars-and-markers-arctest-py

interp_demo.py is an example of using np.interp and not particularly
great at doing that, as one could get nearly the same plot (visually)
without doing interpolation but by using a dashed linestyle... If
anything it belongs to the numpy docs, not the matplotlib docs.
https://matplotlib.org/gallery/lines_bars_and_markers/interp_demo.html#sphx-glr-gallery-lines-bars-and-markers-interp-demo-py

PR Summary

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