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
523027553,MDU6SXNzdWU1MjMwMjc1NTM=,3534,Label row and column titles with units,6815953,open,0,,,3,2019-11-14T18:23:25Z,2019-11-17T17:46:12Z,,NONE,,,,"According to [this plotting example](http://xarray.pydata.org/en/stable/plotting.html#datasets), xlabels and ylabels are labeled with units. However, row labels and column labels omit the units. 

#### MCVE Code Sample
<!-- In order for the maintainers to efficiently understand and prioritize issues, we ask you post a ""Minimal, Complete and Verifiable Example"" (MCVE): http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports -->

```python
import xarray as xr
ds = xr.tutorial.scatter_example_dataset()
ds.plot.scatter(x='A', y='B', col='x', row='z', hue='w', hue_style='discrete')
```

#### Expected Output
Should `xr.FacetGrid` include the functionality for plotting row labels and column labels when provided? 
![image](https://user-images.githubusercontent.com/6815953/68883538-8971da80-0711-11ea-9dcd-a59ea063e2a7.png)


#### Problem Description
<!-- this should explain why the current behavior is a problem and why the expected output is a better solution -->

It could be solved with the following included in [plot/facetgrid.py#L486](https://github.com/pydata/xarray/blob/c0ef2f616e87e9f924425bcd373ac265f14203cb/xarray/plot/facetgrid.py#L486):
```python
if self.data[self._row_var].attrs.get('units'):
   units =  self.data[self._row_var].attrs['units']   
   template=""{coord} = {value} [{units}]""

title = nicetitle(coord=self._row_var, value=row_name, \
                          maxchar=maxchar, template = template, units = unit
```
and with the following included in [plot/facetgrid.py#L499](https://github.com/pydata/xarray/blob/c0ef2f616e87e9f924425bcd373ac265f14203cb/xarray/plot/facetgrid.py#L499):
```python
if self.data[self._col_var].attrs.get('units'):
   units =  self.data[self._col_var].attrs['units']   
   template=""{coord} = {value} [{units}]""

title = nicetitle(coord=self._col_var, value=col_name, \
                          maxchar=maxchar, template = template, units = units)
```
and with the modification of `_nicetitle`:
```python
def _nicetitle(coord, value, maxchar, template, units = None):
    """"""
    Put coord, value in template and truncate at maxchar
    """"""
    prettyvalue = format_item(value, quote_strings=False)
    if units is not None:
        title = template.format(coord=coord, value=prettyvalue, units = units)
    else:
        title = template.format(coord=coord, value=prettyvalue)

    if len(title) > maxchar:
        title = title[: (maxchar - 3)] + ""...""

    return title
```

#### Output of ``xr.show_versions()``
<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.8 (default, Oct  7 2019, 12:59:55) 
[GCC 8.3.0]
python-bits: 64
OS: Linux
OS-release: 4.15.0-65-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.2
libnetcdf: 4.6.3

xarray: 0.14.0
pandas: 0.24.2
numpy: 1.17.3
scipy: 1.2.1
netCDF4: 1.5.1.2
pydap: installed
h5netcdf: 0.7.4
h5py: 2.9.0
Nio: None
zarr: 2.3.2
cftime: 1.0.3.4
nc_time_axis: 1.2.0
PseudoNetCDF: None
rasterio: 1.0.5
cfgrib: 0.9.6.2
iris: None
bottleneck: 1.2.1
dask: 2.6.0
distributed: 2.6.0
matplotlib: 3.0.3
cartopy: 0.16.0
seaborn: 0.9.0
numbagg: None
setuptools: 41.0.1
pip: 19.3.1
conda: None
pytest: 4.4.1
IPython: 7.1.1
sphinx: 2.0.1
</details>
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