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plot_evoked(gfp=True) with single-channel data behaves incorrectly #8772

@hoechenberger

Description

@hoechenberger

Problem

Plotting the GFP of data with only a single sensor via plot_evoked(gfp=True) produces a GFP, although the GFP – describing the spatial standard deviation – should not exist.

Steps to reproduce

import os
import mne
sample_data_folder = mne.datasets.sample.data_path()
sample_data_raw_file = os.path.join(sample_data_folder, 'MEG', 'sample',
                                    'sample_audvis_raw.fif')
raw = mne.io.read_raw_fif(sample_data_raw_file, verbose=False)
events = mne.find_events(raw, stim_channel='STI 014')
event_dict = {'auditory/left': 1, 'auditory/right': 2, 'visual/left': 3,
              'visual/right': 4}
epochs = mne.Epochs(raw, events, tmin=-0.3, tmax=0.7, event_id=event_dict,
                    preload=True)
evoked = epochs['auditory/left'].average()

evoked.pick('EEG 001').plot(gfp=True)

Expected results

  • either no GFP trace, or
  • a GFP constantly at zero, or
  • a warning, or
  • an error message

Actual results

GFP is displayed as the absolute value of the sensor signal.
Screen Shot 2021-01-22 at 16 27 48

Additional information

Using master.

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