How can I solve a mathplotlib error on BigSur?
This issue is not related to Big Sur pre-release version but rather broken matplotlib after Python 3.9 update, the problem exists on Catalina (10.15.7), too:
https://github.com/matplotlib/matplotlib/issues/18678
Most likely missing wheels support in NumPy is causing the issue:
https://github.com/numpy/numpy/issues/17482
Installing older Python version should fix the problem until the problem is fixed on NumPy side. (The current 3.8.x version is 3.8.6).
To install specific Python version please see:
How to install specific version of python on OS X