title: Plot number of occurrences from Pandas DataFrame
body: <p>I have a DataFrame with two columns. One of them is containing timestamps and another one - id of some action. Something like that:</p>
  
  <pre><code>2000-12-29 00:10:00     action1
  2000-12-29 00:20:00     action2
  2000-12-29 00:30:00     action2
  2000-12-29 00:40:00     action1
  2000-12-29 00:50:00     action1
  ...
  2000-12-31 00:10:00     action1
  2000-12-31 00:20:00     action2
  2000-12-31 00:30:00     action2
  </code></pre>
  
  <p>I would like to know how many actions of certain type have been performed in a certain day.
  I.e. for every day, I need to count the number of occurrences of actionX and plot this data with date on X axis and number of occurrences of actionX on Y axes, for each date.</p>
  
  <p>Of course I can count actions for each day naively just by iterating through my dataset. But what's the "right way" to do in with pandas/matplotlib?</p>