FIX: correct sensor name ordering in plot_topomap when using Info object (mne-tools#13686)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: Famous077

doc/changes/dev/13686.bugfix.rst

@@ -0,0 +1 @@
+Fix sensor name ordering in :func:`~mne.viz.plot_topomap` when passing an :class:`~mne.Info` object as ``pos`` with ``names`` argument, by `Famous077`_.

mne/viz/tests/test_topomap.py

@@ -993,3 +993,29 @@ def test_plot_ch_adjacency():
     msg = "Editing a 3d adjacency plot is not supported."
     with pytest.raises(ValueError, match=msg):
         plot_ch_adjacency(info, adj, ch_names, kind="3d", edit=True)
+
+
+@pytest.mark.parametrize("ch_type", ("mag", "grad"))
+def test_plot_topomap_info_names_ordering(ch_type):
+    """Regression test for GH-12700.
+
+    plot_topomap() must preserve correct sensor name ordering when
+    passing an Info object as pos with a names argument.
+    This must be tested with MEG data including gradiometers, since the
+    bug only affected the gradiometer code path.
+    """
+    evoked = read_evokeds(evoked_fname, baseline=(None, 0))[0]
+    evoked = evoked.copy().crop(0, 0).pick(picks=ch_type)
+    info = evoked.info
+    data = evoked.data[:, 0]
+    names = info["ch_names"]
+    if ch_type == "grad":
+        # grad pairs are merged so only every other name is displayed
+        expected_names = names[::2]
+    else:
+        expected_names = names
+    im, _ = plot_topomap(data, info, names=names, show=False)
+    displayed_names = [t.get_text() for t in im.axes.texts]
+    assert displayed_names == list(expected_names), (
+        f"Expected {list(expected_names)}, got {displayed_names}"
+    )

mne/viz/topomap.py

@@ -1292,6 +1292,8 @@ def _plot_topomap(
             pos = _find_topomap_coords(pos, picks=picks[::2], sphere=sphere)
             data, _ = _merge_ch_data(data[picks], ch_type, [])
             data = data.reshape(-1)
+            if names is not None:
+                names = [names[p] for p in picks[::2]]
         else:
             picks = list(range(data.shape[0]))
             pos = _find_topomap_coords(pos, picks=picks, sphere=sphere)