Hi @08-04!
As you mentioned above, QIIME 2 is calling matplotlib, and it is matplotlib that is attempting to use a display, not QIIME 2 (which is why I suggested you edit your matplotlib config).
matplotlib default config varies from installation to installation, it depends on what libraries are available at install time (e.g. qt). Some environments (such as an academic HPC) might have their own matplotlib configs, so we generally steer clear from forcing all of our users to use our config for third party tools. The example I linked to above recommends using Agg as the backend, but there might be other users out there who prefer to use a different offscreen render (or a custom render even!). Similarly, matplotlib also supports per-project configs, which we also don't want to interfere with. Anyway, we will support some form of general QIIME 2 config in the future --- perhaps it is worth continuing this discussion then, to see if there is some way for us to provide a fallback config for tools like matplotlib. Thanks!