fix "auto" scalings for flat or mostly-zero channels (#13376)

Author: drammock

doc/changes/dev/13376.bugfix.rst

@@ -0,0 +1 @@
+Ensure auto-computed plot scalings are always non-zero, by `Daniel McCloy`_.

mne/viz/tests/test_utils.py

@@ -129,6 +129,10 @@ def test_auto_scale():
     raw = read_raw_fif(raw_fname)
     epochs = Epochs(raw, read_events(ev_fname))
     rand_data = np.random.randn(10, 100)
+    # make a stim channel all zeros (gh 13376)
+    ix = raw.get_channel_types().index("stim")
+    raw.load_data()
+    raw._data[ix] = 0.0
 
     for inst in [raw, epochs]:
         scale_grad = 1e10
@@ -142,6 +146,8 @@ def test_auto_scale():
         scalings_new = _compute_scalings(scalings_def, inst)
         assert scale_grad == scalings_new["grad"]
         assert scalings_new["eeg"] != "auto"
+        # make sure an all-zero channel doesn't cause scaling=0 (gh 13376)
+        assert scalings_new["stim"] > 0
 
     with pytest.raises(ValueError, match="Must supply either Raw or Epochs"):
         _compute_scalings(scalings_def, rand_data)

mne/viz/utils.py

@@ -1423,6 +1423,8 @@ def _compute_scalings(scalings, inst, remove_dc=False, duration=10):
         this_data = this_data[np.isfinite(this_data)]
         if this_data.size:
             iqr = np.diff(np.percentile(this_data, [25, 75]))[0]
+            if iqr == 0:  # e.g. sparse stim channels, flat channels
+                iqr = 1.0
         else:
             iqr = 1.0
         scalings[key] = iqr