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
404383025,MDU6SXNzdWU0MDQzODMwMjU=,2725,Line plot with x=coord putting wrong variables on axes,35968931,closed,0,,,3,2019-01-29T16:43:18Z,2019-01-30T02:02:22Z,2019-01-30T02:02:22Z,MEMBER,,,,"When I try to plot the values in a 1D DataArray against the values in one of its coordinates, it does not behave at all as expected: 

```python
import numpy as np
import matplotlib.pyplot as plt
from xarray import DataArray

current = DataArray(name='current', data=np.array([5, 8, 14, 22, 30]), dims=['time'],
                    coords={'time': (['time'], np.array([0.1, 0.2, 0.3, 0.4, 0.5])),
                            'voltage': (['time'], np.array([100, 200, 300, 400, 500]))})

print(current)

# Try to plot current against voltage
current.plot.line(x='voltage')
plt.show()
```
Output:

```
<xarray.DataArray 'current' (time: 5)>
array([ 5,  8, 14, 22, 30])
Coordinates:
  * time     (time) float64 0.1 0.2 0.3 0.4 0.5
    voltage  (time) int64 100 200 300 400 500
```
![incorrect_current_plot](https://user-images.githubusercontent.com/35968931/51924149-683f3800-23e4-11e9-8957-81d32da43117.png)

#### Problem description

Not only is `'voltage'` not on the x axis, but `'current'` isn't on the y axis either!

#### Expected Output

Based on the documentation (and common sense) I would have expected it to plot voltage on the x axis and current on the y axis.

(using a branch of xarray which is up-to-date with master)
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