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
540451721,MDExOlB1bGxSZXF1ZXN0MzU1MjU4NjMy,3646,[WIP] GroupBy plotting,2448579,open,0,,,7,2019-12-19T17:26:39Z,2022-06-09T14:50:17Z,,MEMBER,,1,pydata/xarray/pulls/3646,"<!-- Feel free to remove check-list items aren't relevant to your change -->

 - [x] Tests added
 - [x] Passes `black . && mypy . && flake8`
 - [ ] Fully documented, including `whats-new.rst` for all changes and `api.rst` for new API

This adds plotting methods to GroupBy objects so that it's easy to plot each group as a facet. I'm finding this super helpful in my current research project. 

It's pretty self-contained, mostly just adding `map_groupby*` methods to `FacetGrid`. But that's because I make `GroupBy` mimic the underlying `DataArray` by adding `coords`, `attrs` and `__getitem__`. 

This still needs more tests but I would like feedback on the feature and the implementation.

## Example

``` python
import numpy as np
import xarray as xr

time = np.arange(80)
da = xr.DataArray(5 * np.sin(2*np.pi*time/10), coords={""time"": time}, dims=""time"")
da[""period""] = da.time.where((time % 10) == 0).ffill(""time"")/10
da.plot()
```
![image](https://user-images.githubusercontent.com/2448579/71194665-49f45c00-2284-11ea-96e5-9a5daec1b3a9.png)

``` python
da.groupby(""period"").plot(col=""period"", col_wrap=4)
```
![image](https://user-images.githubusercontent.com/2448579/107123905-a1290780-685d-11eb-9bae-831a7513aaed.png)


``` python
da = da.expand_dims(y=10)
da.groupby(""period"").plot(col=""period"", col_wrap=4, sharex=False, sharey=True, robust=True)
```
![image](https://user-images.githubusercontent.com/2448579/71194716-5c6e9580-2284-11ea-832a-c4e7d9296390.png)

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