title: Python pandas dataframe sort_values does not work
body: <p>I have the following pandas data frame which I want to sort by 'test_type'</p>
  
  <pre><code>  test_type         tps          mtt        mem        cpu       90th
  0  sso_1000  205.263559  4139.031090  24.175933  34.817701  4897.4766
  1  sso_1500  201.127133  5740.741266  24.599400  34.634209  6864.9820
  2  sso_2000  203.204082  6610.437558  24.466267  34.831947  8005.9054
  3   sso_500  189.566836  2431.867002  23.559557  35.787484  2869.7670
  </code></pre>
  
  <p>My code to load the dataframe and sort it is, the first print line prints the data frame above.</p>
  
  <pre><code>        df = pd.read_csv(file) #reads from a csv file
          print df
          df = df.sort_values(by=['test_type'], ascending=True)
          print '\nAfter sort...'
          print df
  </code></pre>
  
  <p>After doing the sort and printing the dataframe content, the data frame still looks like below.</p>
  
  <p>Program output:</p>
  
  <pre><code>After sort...
    test_type         tps          mtt        mem        cpu       90th
  0  sso_1000  205.263559  4139.031090  24.175933  34.817701  4897.4766
  1  sso_1500  201.127133  5740.741266  24.599400  34.634209  6864.9820
  2  sso_2000  203.204082  6610.437558  24.466267  34.831947  8005.9054
  3   sso_500  189.566836  2431.867002  23.559557  35.787484  2869.7670
  </code></pre>
  
  <p>I expect row 3 (test type: sso_500 row) to be on top after sorting. Can someone help me figure why it's not working as it should?</p>