title: How to fill the missing record of Pandas dataframe in pythonic way?
body: <p>I have a Pandas dataframe 'df' like this :</p>
  
  <pre><code>         X   Y  
  IX1 IX2
  A   A1  20  30
      A2  20  30
      A5  20  30
  B   B2  20  30
      B4  20  30
  </code></pre>
  
  <p>It lost some rows, and I want to fill in the gap in the middle like this:</p>
  
  <pre><code>         X   Y  
  IX1 IX2
  A   A1  20  30
      A2  20  30
      A3  NaN NaN
      A4  NaN NaN
      A5  20  30
  B   B2  20  30
      B3  NaN NaN
      B4  20  30
  </code></pre>
  
  <p>Is there a pythonic way to do this ? </p>