issues: 146182176

This data as json

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
146182176 MDExOlB1bGxSZXF1ZXN0NjU0MDc4NzA= 818 Multidimensional groupby 1197350 closed 0     61 2016-04-06T04:14:37Z 2016-07-31T23:02:59Z 2016-07-08T01:50:38Z MEMBER   0 pydata/xarray/pulls/818

Many datasets have a two dimensional coordinate variable (e.g. longitude) which is different from the logical grid coordinates (e.g. nx, ny). (See #605.) For plotting purposes, this is solved by #608. However, we still might want to split / apply / combine over such coordinates. That has not been possible, because groupby only supports creating groups on one-dimensional arrays.

This PR overcomes that issue by using stack to collapse multiple dimensions in the group variable. A minimal example of the new functionality is

``` python

da = xr.DataArray([[0,1],[2,3]], coords={'lon': (['ny','nx'], [[30,40],[40,50]] ), 'lat': (['ny','nx'], [[10,10],[20,20]] )}, dims=['ny','nx']) da.groupby('lon').sum() <xarray.DataArray (lon: 3)> array([0, 3, 3]) Coordinates: * lon (lon) int64 30 40 50 ```

This feature could have broad applicability for many realistic datasets (particularly model output on irregular grids): for example, averaging non-rectangular grids zonally (i.e. in latitude), binning in temperature, etc.

If you think this is worth pursuing, I would love some feedback.

The PR is not complete. Some items to address are - [x] Create a specialized grouper to allow coarser bins. By default, if no grouper is specified, the GroupBy object uses all unique values to define the groups. With a high resolution dataset, this could balloon to a huge number of groups. With the latitude example, we would like to be able to specify e.g. 1-degree bins. Usage would be da.groupby('lon', bins=range(-90,90)). - [ ] Allow specification of which dims to stack. For example, stack in space but keep time dimension intact. (Currently it just stacks all the dimensions of the group variable.) - [x] A nice example for the docs.

 
{
    "url": "https://api.github.com/repos/pydata/xarray/issues/818/reactions",
    "total_count": 2,
    "+1": 2,
    "-1": 0,
    "laugh": 0,
    "hooray": 0,
    "confused": 0,
    "heart": 0,
    "rocket": 0,
    "eyes": 0
}
     13221727 pull

Links from other tables