Looks that matplotlib in Qiime2 2020.2 is containing a matplotlib version with broken heatmaps (at least in seaborn).
Here the link to the issue https://github.com/matplotlib/matplotlib/issues/14675
When I am plotting a heatmap with seaborn inside Qiime2-2020.2, in one of the axes I am reproducing same issue and outside of Qiime2 environment everything is working fine.
Maybe it is worth to update or downgrade current matplotlib version in Qiime2.
Do you have a specific visualization in mind? We have already spent quite a bit of time addressing these issues, here are a few PRs, but could’ve missed something else in the ecosystem:
I think that it is the issue as in your first link. But I got it by building a heatmap by myself using a matplotlib and seaborn inside of the qiime environment (not by running any specific Qiime2 command).
Just wanted to report, that builded matplotlib version in Qiime2 can cause such problems (the same code is working fine outside of Qiime2, so I quess the problem in matplotlib version)