BUG: flat map with "both" on top of one another

[kingjr]

Using stc.plot(surface='flat', hemi='both') leads to a superposition of the two hemispheres:

(in addition, the conventional orientation of flat map is rotated 90* compared to what mne plots:

)

[larsoner]

1. We should just not allow hemi='both' when using a flatmap and tell people to use hemi='split' (ValueError)
2. We should fix the orientation, it's a regression since https://mne.tools/dev/auto_tutorials/source-modeling/plot_visualize_stc.html is wrong but https://mne.tools/stable/auto_tutorials/source-modeling/plot_visualize_stc.html is correct (can you confirm @kingjr ?)

[larsoner]

@kingjr if you want to take a stab at the ori fix at least, it should be that this line needs an appropriate roll= in degrees:

https://github.com/mne-tools/mne-python/blob/main/mne/viz/_brain/view.py#L41

[larsoner]

... although I guess we could allow hemi='both' and add an appropriate offset so they show up in the correct locations relative to one another. This might actually be preferable to hemi='split' in some situations. So I think I changed my mind on (1) above.

[kingjr]

1. I vote for an offset, it's easier to handle zoom in some cases.
2. hard to tell for an auditory response! I would only confirm for a visual one (alternatively, a quick check can be done by using the highlight by area option made by @GuillaumeFavelier)

it's hard for me to do PR on anything related to pyvista, because I only have it working on a headless, and I'm experiencing some painful lag

[GuillaumeFavelier]

No worries, I can add that to my todo list and update #7162

[larsoner]

@GuillaumeFavelier can you take a look?

[GuillaumeFavelier]

I'm on it 👍