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
276185101,MDExOlB1bGxSZXF1ZXN0MTU0MjQxMDUx,1737,WIP: 1d+2d coord plotting,2448579,closed,0,,,3,2017-11-22T19:43:34Z,2017-12-19T23:49:39Z,2017-11-29T11:50:09Z,MEMBER,,0,pydata/xarray/pulls/1737," - [ ] Closes #xxxx
 - [x] Tests added / passed
 - [x] Passes ``git diff upstream/master **/*py | flake8 --diff``
 - [x] Fully documented, including `whats-new.rst` for all changes and `api.rst` for new API

This PR teaches `plot.contourf()` to contour variables with both a 1D co-ordinate (e.g. time) and a 2D co-ordinate (e.g. time-varying depth) `da`:
```
<xarray.DataArray 'S' (z: 5, time: 85646)>
array([[       nan,        nan,        nan, ...,        nan,        nan,
               nan],
       [ 35.02816 ,  34.729567,  34.779223, ...,  34.57513 ,  34.671975,
         34.334675],
       [ 35.206943,  35.163239,  34.938674, ...,  34.780728,  34.836331,
         34.70386 ],
       [       nan,  35.184057,  35.10592 , ...,  34.656925,  34.776915,
         34.429442],
       [ 34.85562 ,  34.81994 ,  35.00963 , ...,  35.014522,  34.9747  ,
         35.033848]])
Coordinates:
    depth    (z, time) float16 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 53.5 ...
  * time     (time) datetime64[ns] 2013-12-19T06:00:01 2013-12-19T06:10:01 ...
Dimensions without coordinates: z
```

Now we can do `da.plot(x='time', y='depth')`

Couple of questions:
1. I've added a test, but am not sure how to work in an assert statement.
2. How do I test that I haven't messed up the syntax in `whats-new.rst`




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