title: move column in pandas dataframe
body: <p>I have the following dataframe:</p>
  
  <pre><code>   a  b   x  y
  0  1  2   3 -1
  1  2  4   6 -2
  2  3  6   9 -3
  3  4  8  12 -4
  </code></pre>
  
  <p>How can I move columns b and x such that they are the last 2 columns in the dataframe? I would like to specify b and x by name, but not the other columns.</p>