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
479914290,MDExOlB1bGxSZXF1ZXN0MzA2NzExNDYx,3210,sparse=True option for from_dataframe and from_series,1217238,closed,0,,,5,2019-08-13T01:09:19Z,2019-08-27T16:04:13Z,2019-08-27T08:54:26Z,MEMBER,,0,pydata/xarray/pulls/3210,"Fixes https://github.com/pydata/xarray/issues/3206

Example usage:

    In [3]: import pandas as pd
       ...: import numpy as np
       ...: import xarray
       ...: df = pd.DataFrame({
       ...:     'w': range(10),
       ...:     'x': list('abcdefghij'),
       ...:     'y': np.arange(0, 100, 10),
       ...:     'z': np.ones(10),
       ...: }).set_index(['w', 'x', 'y'])
       ...:

    In [4]: ds = xarray.Dataset.from_dataframe(df, sparse=True)

    In [5]: ds.z.data
    Out[5]: <COO: shape=(10, 10, 10), dtype=float64, nnz=10, fill_value=nan>

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

 - [x] Closes #3206, Closes #2139
 - [x] Tests added
 - [x] Passes `black . && mypy . && flake8`
 - [x] Fully documented, including `whats-new.rst` for all changes and `api.rst` for new API
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