Update groupby.rst

Author: mazieres

doc/source/groupby.rst

@@ -722,4 +722,29 @@ 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::
+   
+   In [2]: dat = {'a':[1,0,0], 'b':[0,1,0], 'c':[1,0,0], 'd':[2,3,4]}
+   
+   In [3]: df = pd.DataFrame(dat)
+   
+   In [4]: df
+   Out[4]: 
+      a  b  c  d
+   0  1  0  1  2
+   1  0  1  0  3
+   2  0  0  0  4
+   
+   In [5]: df.groupby(df.sum(), axis=1).sum()
+   Out[5]: 
+      1  9
+   0  2  2
+   1  1  3
+   2  0  4