[ENH, MRG] More intuitive colormaps for EEG bridging tutorial

examples/preprocessing/eeg_bridging.py

@@ -35,6 +35,7 @@
 
 import numpy as np
 import matplotlib.pyplot as plt
+from matplotlib.colors import LinearSegmentedColormap
 
 import mne
 
@@ -236,13 +237,25 @@
 # 04-modality-specific-files/03-electroencephalography.html\
 # #electrodes-description-_electrodestsv>`_ file.
 # Since the impedances are not stored for this dataset, we will fake
-# them to demonstrate how they would be plotted.
+# them to demonstrate how they would be plotted. Here, the impedances
+# are plotted as is typical at the end of a setup; most channels are
+# good but there are a few that need to have their impedance lowered.
+# The impedances should ideally all be less than 25 KOhm before
+# starting an experiment when using active systems and less than 5 KOhm
+# when using a passive system.
 
 rng = np.random.default_rng(11)  # seed for reproducibility
 raw = raw_data[1]
-impedances = rng.random((len(raw.ch_names,))) * 10
+# typically impedances < 25 kOhm are acceptable for active systems and
+# impedances < 5 kOhm are desirable for a passive system
+impedances = rng.random((len(raw.ch_names,))) * 30
+impedances[10] = 80  # set a few bad impendances
+impedances[25] = 99
+cmap = LinearSegmentedColormap.from_list(name='impedance_cmap',
+                                         colors=['g', 'y', 'r'], N=256)
 fig, ax = plt.subplots(figsize=(5, 5))
-im, cn = mne.viz.plot_topomap(impedances, raw.info, axes=ax)
+im, cn = mne.viz.plot_topomap(impedances, raw.info, axes=ax,
+                              cmap=cmap, vmin=25, vmax=75)
 ax.set_title('Electrode Impendances')
 cax = fig.colorbar(im, ax=ax)
 cax.set_label(r'Impedance (k$\Omega$)')

mne/viz/topomap.py

@@ -965,6 +965,8 @@ def _plot_topomap(data, pos, vmin=None, vmax=None, cmap=None, sensors=True,
     vmin, vmax = _setup_vmin_vmax(data, vmin, vmax, norm)
     if cmap is None:
         cmap = plt.get_cmap('Reds' if norm else 'RdBu_r')
+    elif isinstance(cmap, str):
+        cmap = plt.get_cmap(cmap)
 
     outlines = _make_head_outlines(sphere, pos, outlines, (0., 0.))
     assert isinstance(outlines, dict)
@@ -2687,6 +2689,7 @@ def plot_bridged_electrodes(info, bridged_idx, ed_matrix, title=None,
     else:
         topomap_args = topomap_args.copy()  # don't change original
     topomap_args.setdefault('image_interp', 'voronoi')
+    topomap_args.setdefault('cmap', 'summer_r')
     topomap_args.setdefault('names', info.ch_names)
     topomap_args.setdefault('show_names', True)
     topomap_args.setdefault('contours', False)