Yeah, using the human-readable `mne.fiff.FIFF` values is better.

I like the idea of having vmin, vmax arguments. Maybe 1) `dict()` support for different channel types, 2) function support, or 3) None (default) leading to old behavior (peak-to-peak scaling, IIRC). Not sure if the parameter would need to be a unified `vlim` or `vmin` and `vmax` separately, though...