Mne-python: topo interactivity broken on evoked.plot()

[jasmainak]

If I do this:

i.e.,

epochs.average().pick_channels(['MEG 1143']).plot()

I get this:

n [15]: Traceback (most recent call last):
  File "/tsi/doctorants/mainak/anaconda/lib/python2.7/site-packages/matplotlib/backends/backend_qt5.py", line 291, in mouseReleaseEvent
    FigureCanvasBase.button_release_event(self, x, y, button)
  File "/tsi/doctorants/mainak/anaconda/lib/python2.7/site-packages/matplotlib/backend_bases.py", line 1891, in button_release_event
    self.callbacks.process(s, event)
  File "/tsi/doctorants/mainak/anaconda/lib/python2.7/site-packages/matplotlib/cbook.py", line 533, in process
    proxy(*args, **kwargs)
  File "/tsi/doctorants/mainak/anaconda/lib/python2.7/site-packages/matplotlib/cbook.py", line 408, in __call__
    return mtd(*args, **kwargs)
  File "/tsi/doctorants/mainak/anaconda/lib/python2.7/site-packages/matplotlib/widgets.py", line 1250, in release
    self.onselect(vmin, vmax)
  File "/tsi/doctorants/mainak/mne-python/mne/viz/evoked.py", line 90, in _butterfly_onselect
    layout=None)
  File "/tsi/doctorants/mainak/mne-python/mne/viz/topomap.py", line 49, in _prepare_topo_plot
    picks, pos = _pair_grad_sensors(info, layout)
  File "/tsi/doctorants/mainak/mne-python/mne/channels/layout.py", line 689, in _pair_grad_sensors
    raise ValueError("No 'grad' channel pairs found.")
ValueError: No 'grad' channel pairs found.

also I find the key mappings a bit weird because it interferes with magnifying the plot ...

[jasmainak]

cc @jaeilepp @jona-sassenhagen

[jona-sassenhagen]

I don't know the interactive functions at all, but is it possibly related to #3003 ?

[jaeilepp]

The problem only occurs if you have grad data where none of the channels have a pair, in which case the error seems reasonable. What I suggest instead is to add the number of channels or pairs plotted on the topomap title, like it is here in evoked.plot.

[jasmainak]

I still don't think it is okay to have errors in interactive mode ...

[jaeilepp]

But you're trying to do something that's not possible. Another option would be to throw a warning, but to me, error seems more appropriate.

[larsoner]

We could disallow interactivity altogether, but then people will wonder why it's there. So to me the only options are warning and error (although maybe we should make a more explicit message in this interactive case, about how it's not possible to select), and I lean toward error. @jasmainak WDYT?

[jasmainak]

okay with error, but maybe we need a better message. Also, it might be better to put it in the text (instead of loading ..., say need corresponding gradiometer channel for interactivity in red font) than in command line?

[larsoner]

Ahh right, yes we could also modify the plot, that would make it very clear.