plot_compare_evokeds should have the same defaults for `combine`

Currently the function mne.viz.plot_compare_evokeds creates two different plots depending on the number of selected channels:

• If one channel is selected (or present in the evoked), it will plot the ERP.
• If two or more channels are selected (or present), it will plot the GFP.

I think this behaviour is potentially misleading, and I would expect the function to have the same defaults for different channels, i.e. mean of the evokeds. As you can see in #10465 , this created quite a bit of confusion for me (probably mainly due to me being new to ERP analysis and not knowing what GFP meant).

I see this has previously already spiked a bit of discussion (e.g. #3393) and was introduced in #6166.
By now, the gfp=... parameter has been removed in favor of the combine=None parameter, which defaults to a different value depending on the present/chosen channels.

I would suggest either

• make combine have the same default for all cases
• make combine an arg, i.e. make users forcefully choose a combination mode

Additionally, the docstring should reflect the importance of the combine parameter.

What are your thoughts on this?

Start at mne.viz.plot_compare_evokeds and inspect how the combine default changes with the selected channel count, along with the function docstring. Review the context in #10465, #3393, and #6166; the issue is complete only after the default behavior or required argument is decided and the documentation reflects its importance.