title: Get keys of pandas.Series.value_counts
body: <p>I use <code>pandas.Series.value_counts</code> to count gender of users. But I also need to get the keys of a result to draw a plot and use keys as a label of the plot.</p>
  
  <p>For example the result of <code>data.gender.value_counts()</code> is:</p>
  
  <pre><code>female     6700
  male       6194
  brand      5942
  unknown    1117
  </code></pre>
  
  <p>And I need to get a list <code>['female', 'male', 'brand', 'unknown']</code> as well and keep the order.</p>
  
  <p>How can I do that? </p>