title: Rolling Mean on pandas on a specific column
body: <p>I have a data frame like this which is imported from a CSV.</p>
  
  <pre><code>              stock  pop
  Date
  2016-01-04  325.316   82
  2016-01-11  320.036   83
  2016-01-18  299.169   79
  2016-01-25  296.579   84
  2016-02-01  295.334   82
  2016-02-08  309.777   81
  2016-02-15  317.397   75
  2016-02-22  328.005   80
  2016-02-29  315.504   81
  2016-03-07  328.802   81
  2016-03-14  339.559   86
  2016-03-21  352.160   82
  2016-03-28  348.773   84
  2016-04-04  346.482   83
  2016-04-11  346.980   80
  2016-04-18  357.140   75
  2016-04-25  357.439   77
  2016-05-02  356.443   78
  2016-05-09  365.158   78
  2016-05-16  352.160   72
  2016-05-23  344.540   74
  2016-05-30  354.998   81
  2016-06-06  347.428   77
  2016-06-13  341.053   78
  2016-06-20  363.515   80
  2016-06-27  349.669   80
  2016-07-04  371.583   82
  2016-07-11  358.335   81
  2016-07-18  362.021   79
  2016-07-25  368.844   77
  ...             ...  ...
  </code></pre>
  
  <p>I wanted to add a new column MA which calculates Rolling mean for the column pop. I tried the following</p>
  
  <pre><code>df['MA']=data.rolling(5,on='pop').mean()
  </code></pre>
  
  <p>I get an error </p>
  
  <pre><code>ValueError: Wrong number of items passed 2, placement implies 1
  </code></pre>
  
  <p>So I thought let me try if it just works without adding a column. I used </p>
  
  <pre><code> data.rolling(5,on='pop').mean()
  </code></pre>
  
  <p>I got the output </p>
  
  <pre><code>               stock  pop
  Date
  2016-01-04       NaN   82
  2016-01-11       NaN   83
  2016-01-18       NaN   79
  2016-01-25       NaN   84
  2016-02-01  307.2868   82
  2016-02-08  304.1790   81
  2016-02-15  303.6512   75
  2016-02-22  309.4184   80
  2016-02-29  313.2034   81
  2016-03-07  319.8970   81
  2016-03-14  325.8534   86
  2016-03-21  332.8060   82
  2016-03-28  336.9596   84
  2016-04-04  343.1552   83
  2016-04-11  346.7908   80
  2016-04-18  350.3070   75
  2016-04-25  351.3628   77
  2016-05-02  352.8968   78
  2016-05-09  356.6320   78
  2016-05-16  357.6680   72
  2016-05-23  355.1480   74
  2016-05-30  354.6598   81
  2016-06-06  352.8568   77
  2016-06-13  348.0358   78
  2016-06-20  350.3068   80
  2016-06-27  351.3326   80
  2016-07-04  354.6496   82
  2016-07-11  356.8310   81
  2016-07-18  361.0246   79
  2016-07-25  362.0904   77
  ...              ...  ...
  </code></pre>
  
  <p>I can't seem to apply Rolling mean on the column pop. What am I doing wrong?</p>