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
268487752,MDU6SXNzdWUyNjg0ODc3NTI=,1661,da.plot.pcolormesh fails when there is a datetime coordinate,500246,closed,0,,,9,2017-10-25T17:44:38Z,2017-10-29T17:28:55Z,2017-10-29T17:28:55Z,CONTRIBUTOR,,,,"
`da.plot.pcolormesh`, where `da` is a `DataArray`, fails with `TypeError: invalid type promotion` when one of the coordinates is a datetime array:

```
$ cat mwe.py
#!/usr/bin/env python3.6

import xarray
import numpy

da = xarray.DataArray(
    numpy.arange(3*4).reshape(3,4),
    dims=(""x"", ""y""),
    coords={""x"": [1,2,3],
            ""y"": [numpy.datetime64(f""2000-01-{x:02d}"")
                  for x in range(1, 5)]})

da.plot.pcolormesh()
$ ./mwe.py
Traceback (most recent call last):
  File ""./mwe.py"", line 13, in <module>
    da.plot.pcolormesh()
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/plot/plot.py"", line 547, in plotmethod
    return newplotfunc(**allargs)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/plot/plot.py"", line 500, in newplotfunc
    **kwargs)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/plot/plot.py"", line 667, in pcolormesh
    primitive = ax.pcolormesh(x, y, z, **kwargs)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/matplotlib/__init__.py"", line 1710, in inner
    return func(ax, *args, **kwargs)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/matplotlib/axes/_axes.py"", line 5636, in pcolormesh
    coords = np.column_stack((X, Y)).astype(float, copy=False)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/numpy/lib/shape_base.py"", line 353, in column_stack
    return _nx.concatenate(arrays, 1)
TypeError: invalid type promotion
```","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1661/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue