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
664568885,MDExOlB1bGxSZXF1ZXN0NDU1Nzg5Mjk2,4259,Improve some error messages: apply_ufunc & set_options.,2448579,closed,0,,,1,2020-07-23T15:23:57Z,2022-01-05T18:57:23Z,2020-07-25T23:04:55Z,MEMBER,,0,pydata/xarray/pulls/4259,"Makes some error messages clearer
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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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