Merge pull request #6304 from mazieres/patch-1

Add example section to groupby.rst and one example

Author: jreback

doc/source/groupby.rst

@@ -722,4 +722,14 @@ To see the order in which each row appears within its group, use the
 
    df.groupby('A').cumcount()
 
-   df.groupby('A').cumcount(ascending=False)  # kwarg only
+   df.groupby('A').cumcount(ascending=False)  # kwarg only
+   
+Examples
+~~~~~~~~
+
+Regroup columns of a DataFrame according to their sum, and sum the aggregated ones.
+
+.. ipython:: python
+   
+   df = pd.DataFrame({'a':[1,0,0], 'b':[0,1,0], 'c':[1,0,0], 'd':[2,3,4]})
+   df.groupby(df.sum(), axis=1).sum()