DataArray coordinates are dropped

Start by running the provided Python reproduction and compare ds["var"].coords with ds.cf["cf_var_name"].coords. Inspect the ds.cf[...] DataArray selection path and verify that coordinates are retained so the shown CF plotting call resolves longitude and latitude successfully.

It looks like when I pull out a DataArray from a Dataset using cf_xarray the coordinates are dropped (while xarray keeps them).
Here is an example:

import xarray as xr
import cf_xarray
import numpy as np

x = 10
y = 20
ds = xr.Dataset(
    dict(
        lon=xr.DataArray(
            np.arange(x), dims=("x"), attrs=dict(standard_name="longitude"),
        ),
        lat=xr.DataArray(
            np.arange(y), dims=("y"), attrs=dict(standard_name="latitude"),
        ),
        var=xr.DataArray(
            np.random.rand(x, y),
            dims=("x", "y"),
            attrs=dict(standard_name="cf_var_name"),
        ),
    )
)
ds = ds.set_coords(["lon", "lat"])

ds.cf.describe()

Axes:
	X: []
	Y: []
	Z: []
	T: []

Coordinates:
	longitude: ['lon']
	latitude: ['lat']
	vertical: []
	time: []

Cell Measures:
	area: unsupported
	volume: unsupported

Standard Names:
	cf_var_name: ['var']

print(ds["var"].coords)

Coordinates:
    lon      (x) int64 0 1 2 3 4 5 6 7 8 9
    lat      (y) int64 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

print(ds.cf["cf_var_name"].coords)

Coordinates:
    *empty*

# OK if I pull out the DataArray directly from ds
ds["var"].cf.plot(x="longitude", y="latitude")

<matplotlib.collections.QuadMesh at 0x7fbf13acf690>

# Problem if I pull out the DataArray from ds.cf
ds.cf["cf_var_name"].cf.plot(x="longitude", y="latitude")

---------------------------------------------------------------------------

KeyError                                  Traceback (most recent call last)

/noc/msm/scratch/climate/malmans/miniconda3/envs/overflows/lib/python3.7/site-packages/xarray/core/dataarray.py in _getitem_coord(self, key)
    628         try:
--> 629             var = self._coords[key]
    630         except KeyError:


KeyError: 'longitude'


During handling of the above exception, another exception occurred:


KeyError                                  Traceback (most recent call last)

<ipython-input-7-072235ade112> in <module>
      1 # Problem if I pull out the DataArray from ds.cf
----> 2 ds.cf["cf_var_name"].cf.plot(x="longitude", y="latitude")


/noc/msm/scratch/climate/malmans/miniconda3/envs/overflows/lib/python3.7/site-packages/cf_xarray/accessor.py in __call__(self, *args, **kwargs)
    629             key_mappers=dict.fromkeys(self._keys, (_get_axis_coord_single,)),
    630         )
--> 631         return self._plot_decorator(plot)(*args, **kwargs)
    632 
    633     def __getattr__(self, attr):


/noc/msm/scratch/climate/malmans/miniconda3/envs/overflows/lib/python3.7/site-packages/cf_xarray/accessor.py in _plot_wrapper(*args, **kwargs)
    597                     xvar = self.accessor[kwargs["x"]]
    598                 else:
--> 599                     xvar = self._obj[kwargs["x"]]
    600                 if "positive" in xvar.attrs:
    601                     if xvar.attrs["positive"] == "down":


/noc/msm/scratch/climate/malmans/miniconda3/envs/overflows/lib/python3.7/site-packages/xarray/core/dataarray.py in __getitem__(self, key)
    638     def __getitem__(self, key: Any) -> "DataArray":
    639         if isinstance(key, str):
--> 640             return self._getitem_coord(key)
    641         else:
    642             # xarray-style array indexing


/noc/msm/scratch/climate/malmans/miniconda3/envs/overflows/lib/python3.7/site-packages/xarray/core/dataarray.py in _getitem_coord(self, key)
    631             dim_sizes = dict(zip(self.dims, self.shape))
    632             _, key, var = _get_virtual_variable(
--> 633                 self._coords, key, self._level_coords, dim_sizes
    634             )
    635 


/noc/msm/scratch/climate/malmans/miniconda3/envs/overflows/lib/python3.7/site-packages/xarray/core/dataset.py in _get_virtual_variable(variables, key, level_vars, dim_sizes)
    169         ref_var = dim_var.to_index_variable().get_level_variable(ref_name)
    170     else:
--> 171         ref_var = variables[ref_name]
    172 
    173     if var_name is None:


KeyError: 'longitude'

PS: This package is great! Let me know if I can help!