title: slicing pandas DataFrame with negative index with ix() method
body: <p>DataFrame.ix() does not seem to slice the DataFrame that I want when negative indexing is used. </p>
  
  <p>I have a DataFrame object and want to slice the last 2 rows.</p>
  
  <pre><code>    In [90]: df = pd.DataFrame(np.random.randn(10, 4))
  
      In [91]: df
      Out[91]: 
              0         1         2         3
      0  1.985922  0.664665 -2.800102  1.695480
      1  0.580509  0.782473  1.032970  1.559917
      2  0.584387  1.798743  0.095950  0.071999
      3  1.956221  0.075530 -0.391008  1.692585
      4 -0.644979 -1.959265  0.749394 -0.437995
      5 -1.204964  0.653912 -1.426602  2.409855
      6  1.178886  2.177259 -0.165106  1.145952
      7  1.410595 -0.761426 -1.280866  0.609122
      8  0.110534 -0.234781 -0.819976  0.252080
      9  1.798894  0.553394 -1.358335  1.278704
  </code></pre>
  
  <p>One way to do it:</p>
  
  <pre><code>    In [92]: df[-2:]
      Out[92]: 
                0         1         2         3
      8  0.110534 -0.234781 -0.819976  0.252080
      9  1.798894  0.553394 -1.358335  1.278704
  </code></pre>
  
  <p>Anther way to do it:</p>
  
  <pre><code>    In [93]: df.ix[len(df)-2:, :]
      Out[93]: 
                0         1         2         3
      8  0.110534 -0.234781 -0.819976  0.252080
      9  1.798894  0.553394 -1.358335  1.278704
  </code></pre>
  
  <p>Now I want to use negative indexing, but having problem:</p>
  
  <pre><code>    In [94]: df.ix[-2:, :]
      Out[94]: 
                0         1         2         3
      0  1.985922  0.664665 -2.800102  1.695480
      1  0.580509  0.782473  1.032970  1.559917
      2  0.584387  1.798743  0.095950  0.071999
      3  1.956221  0.075530 -0.391008  1.692585
      4 -0.644979 -1.959265  0.749394 -0.437995
      5 -1.204964  0.653912 -1.426602  2.409855
      6  1.178886  2.177259 -0.165106  1.145952
      7  1.410595 -0.761426 -1.280866  0.609122
      8  0.110534 -0.234781 -0.819976  0.252080
      9  1.798894  0.553394 -1.358335  1.278704
  </code></pre>
  
  <p>How do I use negative indexing with DataFrame.ix() correctly? Thanks.</p>