title: Dask rolling function by group syntax
body: <p>I struggled for a while with the syntax to work for calculating a rolling function by group for a dask dataframe. The documentation is excellent, but in this case does not have an example. </p>
  
  <p>The working version I have is as follows, from a csv that contains a text field with User ids and and x, y, and z column:</p>
  
  <pre><code>ddf = read_csv('./*.csv')
  ddf.groupby(ddf.User).x.apply(lambda x: x.rolling(5).mean(), meta=('x', 'f8')).compute()
  </code></pre>
  
  <p>Is this the recommended syntax for rolling functions applied by group within dask DataFrames, or is there a recommended alternative?</p>