Describe the new feature or enhancement

mne.viz.plot_compare_evokeds scale the data automatically for each type of sensors. This is not applicable when the data was converted to other scores. Adding a scaling parameter to mne.viz.plot_compare_evokeds is more friendly to users.

Describe your proposed implementation

Adding a scaling parameter to mne.viz.plot_compare_evokeds like that in evoked.plot_topomap.