title: Unpivot Pandas Data
body: <p>I currently have a <code>DataFrame</code> laid out as:</p>
  
  <pre><code>        Jan Feb Mar Apr ...
  2001    1   12  12  19  
  2002    9   ...
  2003    ...
  </code></pre>
  
  <p>and I would like to "unpivot" the data to look like:</p>
  
  <pre><code>Date    Value
  Jan 2001    1
  Feb 2001    1
  Mar 2001    12
  ...
  Jan 2002    9
  </code></pre>
  
  <p>What is the best way to accomplish this using pandas/NumPy?</p>