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
365778456,MDExOlB1bGxSZXF1ZXN0MjE5NjEzMDM3,2453,plot.contour: Don't make cmap if colors is a single color.,2448579,closed,0,,,0,2018-10-02T07:48:43Z,2019-04-12T16:29:45Z,2018-10-02T15:46:00Z,MEMBER,,0,pydata/xarray/pulls/2453," - [x] Tests added (for all bug fixes or enhancements)
 - [x] Tests passed (for all non-documentation changes)
 - [x] Fully documented, including `whats-new.rst` for all changes and `api.rst` for new API (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

By default, matplotlib draws dashed negative contours when a single color is passed to `Axes.contour`. We lost this feature by manually specifying cmap everytime.

Currently on master:
```
delta = 0.025
x = np.arange(-3.0, 3.0, delta)
y = np.arange(-2.0, 2.0, delta)
X, Y = np.meshgrid(x, y)
Z1 = np.exp(-X**2 - Y**2)
Z2 = np.exp(-(X - 1)**2 - (Y - 1)**2)
Z = xr.DataArray((Z1 - Z2) * 2, dims=['y', 'x'], coords={'x': x, 'y': y})

f, ax = plt.subplots(1, 2)
hdl = ax[0].contour(Z, colors='k')
hdlx = Z.plot.contour(ax=ax[1], colors='k')
```
![image](https://user-images.githubusercontent.com/2448579/46335584-e7a14280-c662-11e8-91e2-e1fedebf81dc.png)


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