Thanks @GuySten. I ended up removing all the extra code related to cross_sections. The recommended way to handle the data is to set openmc.config['mg_cross_sections'] before calling the plot() method. I'll comment on the original issue too.

Sorry about the mishaps in my updates. I did make another change to simplify the additions to Model.plot. The reason that the relative path didn't work in the test is because for testing we set openmc.config['resolve_paths'] = False, so the relative path is kept. However, the default configuration for users is openmc.config['resolve_paths'] = True which means that it should work out of the box for users. Instead of introducing config.patch in Model.plot, I changed it so that we set the absolute path in the test. Let me know if you're OK with this now and if so, we're good to merge.