Summary

• In https://matplotlib.org/stable/gallery/event_handling/resample.html, it is shown that one can resample lines by inserting the underlying line coordinates.
• If we want to do the same resampling for fill_between, we need a dedicated function to generate the polygon coordinates only.

Proposed fix

Extract the first few lines in _fill_between_x_or_y in matplotlib.axes._axes until collection = mcoll.PolyCollection(polys, **kwargs) to a separate instance method.