title: Shift column in pandas dataframe up by one?
body: <p>I've got a pandas dataframe. I want to 'lag' one of my columns. Meaning, for example, shifting the entire column 'gdp' up by one, and then removing all the excess data at the bottom of the remaining rows so that all columns are of equal length again.</p>
  
  <pre class="lang-none prettyprint-override"><code>df =
      y  gdp  cap
  0   1    2    5
  1   2    3    9
  2   8    7    2
  3   3    4    7
  4   6    7    7
  
  df_lag =
      y  gdp  cap
  0   1    3    5
  1   2    7    9
  2   8    4    2
  3   3    7    7
  </code></pre>
  
  <p>Anyway to do this?</p>