Repository: mne-tools/mne-python

Issue:
stc.plot with pyvista: error with hemi='both'
I am not able to plot the two hemispheres of an stc at the same time with `time_viewer=True` , for instance when using [this](https://mne.tools/stable/auto_examples/visualization/plot_xhemi.html#sphx-glr-auto-examples-visualization-plot-xhemi-py) example:  

```
stc.plot(hemi='both', subjects_dir=subjects_dir, initial_time=0.07,
          size=(800, 600),time_viewer=True)
```

Error: 
> 
>   File "/home/sh254795/pyvista/pyvista/plotting/plotting.py", line 2215, in subplot
>     raise IndexError('Column index is out of range ({})'.format(self.shape[1]))
> 
> IndexError: Column index is out of range (1)

With `hemi='split'` the panels overlap, and the time-slider is not usable (I can send a screenshot).
Also, from the documentation it is not clear to me what the difference between 'both' and 'split' would do. 





Discussion:
Could you share a screenshot? It would help me understand the situation.

A fix for the slider position in `hemi=split` is in https://github.com/mne-tools/mne-python/pull/7227/commits/5d78f05ef95c858edb20a7851572cc92719eb1fa. The PR is still waiting for reviews but hopefully soon to be merged.

Also I can reproduce the issue with `hemi=both` so I'll work on a fix soon :+1: 

> Also, from the documentation it is not clear to me what the difference between 'both' and 'split' would do.

From the doc of `_Brain`:
```
    hemi : str
        Hemisphere id (ie 'lh', 'rh', 'both', or 'split'). In the case
        of 'both', both hemispheres are shown in the same window.
        In the case of 'split' hemispheres are displayed side-by-side
        in different viewing panes.
```

The hemispheres are in the same view with `both` and are in different views with `split.` Feel free to share suggestions about how we can improve the documentation.
Here is an example:

Split | Both
------|------
![2020-01-24_1920x1080](https://user-images.githubusercontent.com/18143289/73065827-cd692480-3ea4-11ea-82ba-4525d510ed7f.png) | ![2020-01-24_1920x1080](https://user-images.githubusercontent.com/18143289/73065863-e07bf480-3ea4-11ea-9dd8-0a225a807c93.png)


Resolving patch:
diff --git a/mne/viz/_brain/_timeviewer.py b/mne/viz/_brain/_timeviewer.py
--- a/mne/viz/_brain/_timeviewer.py
+++ b/mne/viz/_brain/_timeviewer.py
@@ -164,6 +164,9 @@ def __init__(self, brain):
 
         for hemi in self.brain._hemis:
             ci = 0 if hemi == 'lh' else 1
+            # with both, all hemis are on the same view
+            if self.brain._hemi == 'both':
+                ci = 0
             for ri, view in enumerate(self.brain._views):
                 self.plotter.subplot(ri, ci)
                 name = "orientation_" + str(ri) + "_" + str(ci)
@@ -426,19 +429,6 @@ def set_slider_style(self, slider, show_label=True):
             if not show_label:
                 slider_rep.ShowSliderLabelOff()
 
-            # add support for split window
-            shape = self.plotter.shape
-            pointa = slider_rep.GetPoint1Coordinate().GetValue()
-            pointb = slider_rep.GetPoint2Coordinate().GetValue()
-            pointa = _normalize(pointa, shape)
-            pointb = _normalize(pointb, shape)
-            slider_rep.GetPoint1Coordinate().\
-                SetCoordinateSystemToNormalizedDisplay()
-            slider_rep.GetPoint1Coordinate().SetValue(pointa[0], pointa[1])
-            slider_rep.GetPoint2Coordinate().\
-                SetCoordinateSystemToNormalizedDisplay()
-            slider_rep.GetPoint2Coordinate().SetValue(pointb[0], pointb[1])
-
 
 def _set_text_style(text_actor):
     if text_actor is not None:


Test patch: