id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
453126577,MDU6SXNzdWU0NTMxMjY1Nzc=,3002,plot.pcolormesh fails with shading='gouraud',35968931,closed,0,,,5,2019-06-06T16:27:00Z,2020-11-29T16:28:32Z,2019-06-06T22:26:35Z,MEMBER,,,,"`xarray.plot.pcolormesh()` fails when you pass the `matplotlib.pyplot.pcolormesh()` keyword argument `shading='gouraud'` to it.

#### Code Sample, a copy-pastable example if possible

```python
import matplotlib.pyplot as plt
import numpy as np
import xarray as xr

lon, lat = np.meshgrid(np.linspace(-20, 20, 5), np.linspace(0, 30, 4))
lon += lat/10
lat += lon/10

da = xr.DataArray(np.arange(20).reshape(4, 5), dims=['y', 'x'],
                  coords = {'lat': (('y', 'x'), lat),
                            'lon': (('y', 'x'), lon)})

da.plot.pcolormesh('lon', 'lat', shading='gouraud')
plt.show()
```

#### Problem description

This gives an error:

```
Traceback (most recent call last):
  File ""mwe.py"", line 17, in <module>
    da.plot.pcolormesh('lon', 'lat', shading='gouraud')
  File ""/home/tegn500/Documents/Work/Code/xarray/xarray/plot/plot.py"", line 721, in plotmethod
    return newplotfunc(**allargs)
  File ""/home/tegn500/Documents/Work/Code/xarray/xarray/plot/plot.py"", line 662, in newplotfunc
    **kwargs)
  File ""/home/tegn500/Documents/Work/Code/xarray/xarray/plot/plot.py"", line 864, in pcolormesh
    primitive = ax.pcolormesh(x, y, z, **kwargs)
  File ""/home/tegn500/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/__init__.py"", line 1805, in inner
    return func(ax, *args, **kwargs)
  File ""/home/tegn500/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/axes/_axes.py"", line 5971, in pcolormesh
    X, Y, C = self._pcolorargs('pcolormesh', *args, allmatch=allmatch)
  File ""/home/tegn500/anaconda3/envs/py36/lib/python3.6/site-packages/matplotlib/axes/_axes.py"", line 5559, in _pcolorargs
    C.shape, Nx, Ny, funcname))
TypeError: Dimensions of C (4, 5) are incompatible with X (6) and/or Y (5); see help(pcolormesh)
```

#### Expected Output

This should give almost the same image as in the documentation, just with smoother shading:

![Figure_1](https://user-images.githubusercontent.com/35968931/59049474-f68a6580-887f-11e9-83db-697c38acdf5e.png)

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