title: count of unique values in pandas dataframe column
body: <p>I am trying to find the frequency of unique values in a column of a pandas dataframe I know how to get the unique values like this: </p>
  
  <pre><code>data_file.visiting_states()
  </code></pre>
  
  <p>returns : </p>
  
  <pre><code>array(['CA', 'VA', 'MT', nan, 'CO', 'CT'],    dtype=object)
  </code></pre>
  
  <p>and I want to return the count of those unique values and I know I cant do .value_counts() because its a numpy array </p>