DOC: add interp example

[raybellwaves]

• [NA] Closes #xxxx (remove if there is no corresponding issue, which should only be the case for minor changes)
• [NA] Tests added (for all bug fixes or enhancements)
• [NA] Tests passed (for all non-documentation changes)
• [NA] Fully documented, including whats-new.rst for all changes and api.rst for new API (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

I wanted to add an example as googling 'xarray interp' takes me to http://xarray.pydata.org/en/stable/generated/xarray.Dataset.interp.html.

However, there lots of great examples in http://xarray.pydata.org/en/stable/interpolation.html
Perhaps I should edit it to:

Examples
--------
See http://xarray.pydata.org/en/stable/interpolation.html

I was also thinking about adding a note about interpolating time as it came up in #2284 but not sure what would be the best way to do so.

[fujiisoup]

Thanks for adding this to doc.
It is very welcome if you could add an example for interp with datetime.

To do so, it would be best place to put it in here
https://github.com/pydata/xarray/blob/master/doc/interpolation.rst

[raybellwaves]

@fujiisoup I would like to add another example on how to update a DataArray with the interpolated data. I looked in http://xarray.pydata.org/en/stable/combining.html but couldn't work out how best to do it.

I tried using merge but it requires the DataArray's to be named.

I used concat in this example but it is not correct as I would like the coordinates to be in order (in-place may be the right word).

import xarray as xr
import pandas as pd
import numpy as np

da_dt64 = xr.DataArray([1, 3], [('time', pd.date_range('1/1/2000', '1/3/2000', periods=2))])
a = da_dt64.interp(time=np.datetime64('2000-01-02'))
xr.concat([da_dt64, a], dim='time')
#<xarray.DataArray (time: 3)>
#array([ 1.,  3.,  2.])
#Coordinates:
#  * time     (time) datetime64[ns] 2000-01-01 2000-01-03 2000-01-02

[fujiisoup]

Thanks.

I think you can do

da_dt64.interp(time=pd.date_range('1/1/2000', '1/3/2000', periods=3))

[fujiisoup]

It is currently true, but I think we could support da_dt64.interp(time='2000-01-02') in the future.
Do you mind to send another PR to do this?

[raybellwaves]

I'm trying to wrap up another PR. Unfortunately, I don't want to over promise and may not be able to get to this until a couple of weeks. If you have ideas of how to do this we can move the discussion to #2284

[fujiisoup]

Thanks, @raybellwaves.