title: Left-align a pandas rolling object
body: <p>Using pandas 0.18.1, I'd like to take the rolling average of a one-column dataframe. Since version 0.18.0, this is done with rolling() objects. The default for these rolling objects is to be right-justified. There is a boolean argument you can pass, center=True, to align the rolling object to the center value, but there doesn't seem to be a way to left-align it. Here's an example:</p>
  
  <pre><code>df = pandas.DataFrame({'A': [2,3,6,8,20, 27]})
  df
      A
  0   2
  1   3
  2   6
  3   8
  4  20
  5  27
  </code></pre>
  
  <p>The standard method automatically aligns to the right, so there's no value at the first two indecies with a window of size three:</p>
  
  <pre><code> df.rolling(window=3).mean()
             A
  0        NaN
  1        NaN
  2   3.666667
  3   5.666667
  4  11.333333
  5  18.333333
  </code></pre>
  
  <p>We can center-align the operation like this:</p>
  
  <pre><code>df.rolling(window=3).mean(center=True)
             A
  0        NaN
  1   3.666667
  2   5.666667
  3  11.333333
  4  18.333333
  5        NaN
  </code></pre>
  
  <p>But what I'm looking for is this:</p>
  
  <pre><code>df.rolling(3).mean()
              A
   0   3.666667
   1   5.666667
   2  11.333333
   3  18.333333
   4        NaN
   5        NaN
  </code></pre>
  
  <p>I can accomplish this by doing it with the default right alignment, and then re-indexing it, or by reversing the order of the rows and then doing it "right-aligned" but these are work-arounds for what should be a straight-forward operation.</p>