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
922791001,MDU6SXNzdWU5MjI3OTEwMDE=,5473,Junk slice written to zarr with append_dim option,41342785,closed,0,,,2,2021-06-16T15:51:33Z,2021-10-19T18:51:18Z,2021-10-19T18:51:18Z,CONTRIBUTOR,,,," I often write to zarr using the `append_dim` option. But I find that  sometimes I get a junk slice in the first index of the zarr file along the dimenstion I am appending along. This seems to be related to the way coordinates are assigned along this dimension. Here's a minimal working example:

```
import xarray as xr
import numpy as np

target='test_zarr.zarr'
for i in range(0,10):
    test_slice = xr.DataArray(np.random.rand(10,20),dims=['j','k'])
    test_slice = test_slice.assign_coords({""time"": i*10}) #this line is necessary for this example to fail
    test_slice = test_slice.expand_dims('time')
    test_ds = test_slice.to_dataset(name='u')
    test_ds.to_zarr(target, append_dim='time')
    
```

When I open the `zarr` file:

```
test_opened = xr.open_zarr('test_zarr.zarr')
test_opened.time
```
gives 
```
array([140624802912264,               0,              10,              20,
                    30,              40,              50,              60,
                    70,              80,              90])
```

**Environment**:

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

INSTALLED VERSIONS
------------------
commit: None
python: 3.9.4 | packaged by conda-forge | (default, May 10 2021, 22:13:33) 
[GCC 9.3.0]
python-bits: 64
OS: Linux
OS-release: 3.10.0-1160.11.1.el7.centos.plus.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: None
libnetcdf: None

xarray: 0.18.2
pandas: 1.2.4
numpy: 1.20.3
scipy: 1.6.3
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: 2.8.1
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2021.06.0
distributed: 2021.06.0
matplotlib: 3.4.2
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 49.6.0.post20210108
pip: 21.1.2
conda: None
pytest: None
IPython: 7.24.1
sphinx: None


</details>
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847376965,MDExOlB1bGxSZXF1ZXN0NjA2Mzk1NDky,5100,Tweaked Dataset.plot docs to better describe quiver,41342785,closed,0,,,1,2021-03-31T21:33:48Z,2021-05-01T17:40:06Z,2021-04-01T16:44:23Z,CONTRIBUTOR,,0,pydata/xarray/pulls/5100,"closes #5002

Based on issue #5002, I did a few tweaks to make it clearer which parts of the generated documentation apply to `quiver`, and which apply to `scatter`. I also removed references to `contourf` because that is now documented in a separate location. 
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/5100/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull
484768136,MDExOlB1bGxSZXF1ZXN0MzEwNTc2Mzc2,3259,New feature of filter_by_attrs added to whats-new.rst,41342785,closed,0,,,1,2019-08-24T02:28:05Z,2019-08-24T03:20:59Z,2019-08-24T03:20:59Z,CONTRIBUTOR,,0,pydata/xarray/pulls/3259,"I missed this in my previous pull request. @dcherian

<!-- Feel free to remove check-list items aren't relevant to your change -->

 - [ ] Closes #xxxx
 - [ ] Tests added
 - [ ] Passes `black . && mypy . && flake8`
 - [ ] Fully documented, including `whats-new.rst` for all changes and `api.rst` for new API
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3259/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull
484243962,MDExOlB1bGxSZXF1ZXN0MzEwMTY2MDYz,3247,Update filter_by_attrs to use 'variables' instead of 'data_vars',41342785,closed,0,,,8,2019-08-22T22:23:31Z,2019-08-23T23:27:03Z,2019-08-23T23:26:00Z,CONTRIBUTOR,,0,pydata/xarray/pulls/3247,"This will allow `filter_by_attrs` to filter coordinates as well as variables. @ocefpaf
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3247/reactions"", ""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull
484623632,MDExOlB1bGxSZXF1ZXN0MzEwNDYyOTI1,3253,Updater to testing environment name,41342785,closed,0,,,1,2019-08-23T16:55:49Z,2019-08-23T17:00:40Z,2019-08-23T17:00:40Z,CONTRIBUTOR,,0,pydata/xarray/pulls/3253,"The testing environment name has been updated to `xarray-tests` in the package and we should do this in the documentation as well.

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