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
298481284,MDExOlB1bGxSZXF1ZXN0MTcwMTA4MjE1,1926,"Add x,y kwargs for plot.line().",2448579,closed,0,,,10,2018-02-20T06:04:35Z,2018-05-10T05:12:03Z,2018-03-05T22:14:46Z,MEMBER,,0,pydata/xarray/pulls/1926," - [x] Partially closes #575 
 - [x] Tests added
 - [x] Tests passed
 - [x] Fully documented, including `whats-new.rst` for all changes and `api.rst` for new API

### Description
`plot.line` now supports both 1D and 2D DataArrays as input. I've changed  some variable names to make code clearer:
   1. set `xplt`, `yplt` to be values that are passed to `ax.plot()`
   2. `xlabel`, `ylabel` are axes labels
   3. `xdim`, `ydim` are dimension names

### Example
This code
```z = np.arange(10)
da = xr.DataArray(np.cos(z), dims=['z'], coords=[z], name='f')

xy = [[None, None],
     [None, 'f'],
     [None, 'z'],
     ['f', None],
      ['z', None],
      ['z', 'f'],
      ['f', 'z']]

f, ax = plt.subplots(2,4)

for aa, (x,y) in enumerate(xy):
    da.plot(x=x, y=y, ax=ax.flat[aa])
    ax.flat[aa].set_title('x='+str(x)+ ' | '+'y='+str(y))
```

yields

![image](https://user-images.githubusercontent.com/2448579/36409515-ca2d6d9e-15c0-11e8-9d04-c7c741ac3d5d.png)

### Feedback requested

Should I refactor out the kwarg checking? ","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1926/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull
298672745,MDExOlB1bGxSZXF1ZXN0MTcwMjUyMTky,1928,facetgrid: unset cmap if colors is specified.,2448579,closed,0,,,2,2018-02-20T17:03:45Z,2018-05-10T05:12:03Z,2018-03-06T05:05:59Z,MEMBER,,0,pydata/xarray/pulls/1928," - [x] Closes #1461 
 - [x] Tests added
 - [x] Tests passed
 - [x] Fully documented, including `whats-new.rst` for all changes and `api.rst` for new API

This seemed the cleanest way to fix this. If both `colors` and `cmap` are specified, raise an error. If `colors` is specified and `cmap` was None, I set the auto-inferred `cmap` to None.","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1928/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull