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
664595680,MDU6SXNzdWU2NjQ1OTU2ODA=,4260,use matplotlib's categorical axis features,2448579,closed,0,,,0,2020-07-23T16:01:13Z,2021-06-21T17:45:39Z,2021-06-21T17:45:39Z,MEMBER,,,,"<!-- Please do a quick search of existing issues to make sure that this has not been asked before. -->

**Is your feature request related to a problem? Please describe.**
xarray currently doesn't allow plotting against coordinates with string labels for example.

**Describe the solution you'd like**
Use matplotlib's categorical axis support. Example: https://matplotlib.org/gallery/lines_bars_and_markers/categorical_variables.html

This may be the only place a change is required: https://github.com/pydata/xarray/blob/4e893317240ed1a80e65ea2de107e9179bb65446/xarray/plot/utils.py#L572-L608
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922792799,MDExOlB1bGxSZXF1ZXN0NjcxNjI1NDc3,5474,Refactor out coarsen tests,2448579,closed,0,,,3,2021-06-16T15:52:57Z,2021-06-21T17:04:02Z,2021-06-21T16:35:36Z,MEMBER,,0,pydata/xarray/pulls/5474,"<!-- Feel free to remove check-list items aren't relevant to your change -->

- xref #5409
- [x] Tests added
- [x] Passes `pre-commit run --all-files`

Some questions:
1. flake8 fails with some false positives. What do we do about that? 
2. I am importing the `da` and `ds` fitures from `test_dataarray` and `test_dataset`. Is that the pattern we want to follow?

```
xarray/tests/test_coarsen.py:9:1: F401 '.test_dataarray.da' imported but unused
xarray/tests/test_coarsen.py:10:1: F401 '.test_dataset.ds' imported but unused
xarray/tests/test_coarsen.py:13:36: F811 redefinition of unused 'ds' from line 10
xarray/tests/test_coarsen.py:20:26: F811 redefinition of unused 'ds' from line 10
xarray/tests/test_coarsen.py:35:25: F811 redefinition of unused 'ds' from line 10
xarray/tests/test_coarsen.py:51:5: F811 redefinition of unused 'da' from line 9
xarray/tests/test_coarsen.py:62:5: F811 redefinition of unused 'da' from line 9
xarray/tests/test_coarsen.py:84:5: F811 redefinition of unused 'ds' from line 10
xarray/tests/test_coarsen.py:156:5: F811 redefinition of unused 'ds' from line 10
xarray/tests/test_coarsen.py:186:25: F811 redefinition of unused 'ds' from line 10
xarray/tests/test_coarsen.py:217:5: F811 redefinition of unused 'da' from line 9
xarray/tests/test_coarsen.py:269:5: F811 redefinition of unused 'da' from line 9
```
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