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
292054887,MDU6SXNzdWUyOTIwNTQ4ODc=,1862,Weird looking plots from combined DataArrays,244887,closed,0,,,5,2018-01-26T22:48:04Z,2019-01-22T22:48:54Z,2019-01-22T22:48:54Z,CONTRIBUTOR,,,,"I am trying to figure out how to plot 2 dimensional data from multiple data arrays that share common coordinates. My first guess was to throw the data arrays into a dataset and call a plot method, but it seems that is unsupported (perhaps with good reason). My second idea was to  trying to ""merge"" the data arrays into a single one and then call the plot method. Here is what I get:

```python
N = 2**6
x, y = range(N), range(N)
n = int(N/4)
A, B = np.random.rand(n,n), np.random.rand(n,n)

xrA = xr.DataArray(A, dims=('x','y'), coords={'x':x[:n],'y':y[:n]})
xrB = xr.DataArray(B, dims=('x','y'), coords={'x':x[-n:],'y':y[-n:]})

xrAB = xrA.combine_first(xrB)

dataarrays = [xrA,xrB,xrAB]

fig,axs = plt.subplots(1,3,figsize=(15,4))
for ax,data in zip(axs,dataarrays):
    data.plot(ax=ax)
```

![image](https://user-images.githubusercontent.com/244887/35463660-9aaa9880-02a6-11e8-95f5-2a13c00fb25e.png)

I was not expecting to see the funny looking artifacts extending from the edges of the region with non nan data. A slightly more elaborate version with three non nan regions shows similar behavior:
```
xrC = xr.DataArray(C, dims=('x','y'), coords={'x':x[15:n+15],'y':y[-n-10:-10]})

xrABC = xrA.combine_first(xrB).combine_first(xrC)

fig,ax = plt.subplots(figsize=(5,5))
xrABC.plot(ax=ax)
```

![image](https://user-images.githubusercontent.com/244887/35463861-bb704136-02a7-11e8-9693-79be0b582cf4.png)

Is this a bug or a feature? If not a bug, how should I go about plotting my array data so that nan regions look as such?

#### Output of ``xr.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

xarray: 0.10.0
pandas: 0.20.3
numpy: 1.13.0
scipy: 0.18.1
netCDF4: None
h5netcdf: None
Nio: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.15.3
matplotlib: 2.0.0
cartopy: None
seaborn: 0.8.0
setuptools: 36.5.0.post20170921
pip: 9.0.1
conda: 4.4.6
pytest: 3.2.1
IPython: 6.1.0
sphinx: 1.6.3

</details>
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