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
596249070,MDU6SXNzdWU1OTYyNDkwNzA=,3954,Concatenate 3D array with 2D array,30388627,open,0,,,5,2020-04-08T01:36:28Z,2022-05-06T16:39:22Z,,NONE,,,,"The 3D array has three dims: z, y and x.
The 2D array has two dims: y and x.
When I try to concatenate them by expanding the 2D array with z dim, there's something wrong in `_dataset_concat`

#### MCVE Code Sample
```python
import xarray as xr
import numpy as np

x = 2
y = 4
z = 3
data = np.arange(x*y*z).reshape(z, x, y)

# 3d array with coords
a = xr.DataArray(data, dims=['z', 'y', 'x'], coords={'z': np.arange(z)})

# 2d array without coords
b = xr.DataArray(np.arange(x*y).reshape(x, y)*1.5, dims=['y', 'x'])

# expand 2d to 3d
b = b.expand_dims('z')

# concat
comb = xr.concat([a, b], dim='z')
```

#### Expected Output
Same as `np.concatenate`:
```
concat_array = np.concatenate((a, b))
comb = xr.DataArray(concat_array, dims={'z', 'y', 'x'})
```

```
<xarray.DataArray (z: 4, x: 2, y: 4)>
array([[[ 0. ,  1. ,  2. ,  3. ],
        [ 4. ,  5. ,  6. ,  7. ]],

       [[ 8. ,  9. , 10. , 11. ],
        [12. , 13. , 14. , 15. ]],

       [[16. , 17. , 18. , 19. ],
        [20. , 21. , 22. , 23. ]],

       [[ 0. ,  1.5,  3. ,  4.5],
        [ 6. ,  7.5,  9. , 10.5]]])
Dimensions without coordinates: z, x, y
```

#### Problem Description
```
    comb = xr.concat([a, b], dim='z')
  File ""E:\miniconda3\envs\satpy\lib\site-packages\xarray\core\concat.py"", line 135, in concat
    return f(objs, dim, data_vars, coords, compat, positions, fill_value, join)
  File ""E:\miniconda3\envs\satpy\lib\site-packages\xarray\core\concat.py"", line 455, in _dataarray_concat
    join=join,
  File ""E:\miniconda3\envs\satpy\lib\site-packages\xarray\core\concat.py"", line 395, in _dataset_concat
    raise ValueError(""%r is not present in all datasets."" % k)
ValueError: 'z' is not present in all datasets.
```

As suggested by @dcherian, assigning the coordinate label by changing `b = b.expand_dims('z')` to `b = b.expand_dims(z=[3])` makes it work.

#### Versions

<details><summary>Output of `xr.show_versions()`</summary>

<!-- Paste the output here xr.show_versions() here -->
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.6 | packaged by conda-forge | (default, Jan  7 2020, 21:48:41) [MSC v.1916 64 bit (AMD64)]
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
libhdf5: None
libnetcdf: None

xarray: 0.15.1
pandas: 1.0.3
numpy: 1.18.1
scipy: 1.4.1
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: 1.1.1.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: 1.1.3
cfgrib: None
iris: None
bottleneck: None
dask: 2.10.1
distributed: 2.14.0
matplotlib: 3.2.1
cartopy: 0.17.0
seaborn: 0.10.0
numbagg: None
setuptools: 46.1.3.post20200325
pip: 20.0.2
conda: None
pytest: None
IPython: 7.13.0
sphinx: 2.4.4

</details>
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1086038682,I_kwDOAMm_X85Au6Ka,6095,Issue on page /examples/multidimensional-coords.html,30388627,open,0,,,1,2021-12-21T17:04:09Z,2021-12-21T19:17:42Z,,NONE,,,,The `Groupby` example focuses on lat_bin. Then the `xc` in the `groupby_bins` function should be `yc`.,"{""url"": ""https://api.github.com/repos/pydata/xarray/issues/6095/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue