title: sorting rows in a pandas dataframe in a way which is not alphabetical
body: <p>I have some dataframes (df) with categorical data starting with: a, b, c and a category for "remaining categories". </p>
  
  <p>I would like to sort the month column in the dataframe ascending=true, but then have the category column sorted so that they are in the following order:</p>
  
  <pre><code>c
  
  a
  
  b
  </code></pre>
  
  <p>"remaining category"</p>
  
  <p>Is this possible? --> Basically I want a custom sort order for a specific column, but then have the month column sorted in order of date.</p>