title: Pandas plot DataFrame rolling mean gives weird result
body: <p>If I have a DataFrame built as </p>
  
  <pre><code>df = pd.DataFrame({'a': [i for i in range(10)],  
                     'b': [2*i for i in range(10)]})
  </code></pre>
  
  <p>so that its plot of 'b' against 'a' (obtained via <code>df.plot('a', 'b')</code>) is simply a straight line, and I compute the running mean of it as </p>
  
  <pre><code>rm_df = df.rolling(window=2).mean()
  </code></pre>
  
  <p>and try to plot again 'b' against 'a', I get the reported figure, where the x axis is giving weird values.</p>
  
  <p><a href="https://i.stack.imgur.com/oMdZy.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/oMdZy.png" alt="weird values in rolling mean df plotting"></a></p>
  
  <p>I've checked by looking at the dataframe that all values are good, so what is happening? </p>