title: Scatter plot a DataFrame grouped by 2 columns and having an aggregation
body: <p>I am trying to do a scatter plot over the number of items in the DataFrame for each date/time combination. I've grouped data like this:</p>
  
  <pre><code>dff = pd.DataFrame(df.groupby(['date', 'time']).size().rename('count'))
  </code></pre>
  
  <p>and it looks like this:</p>
  
  <pre><code>                           count
  date         time       
  2017-05-19   15:00         1
               15:30         1
               16:00         1
               16:30         1
               17:00         1
  2017-05-23   10:00         2
               10:30         2
               11:00         2
  ...
  </code></pre>
  
  <p>Now, how can I scatter plot the counts having <code>date</code>s on the X axis and <code>time</code>s on the Y axis? <code>plt.scatter(x, y, s=area, c=colors)</code> is the signature, but however I try to select <code>x</code> and <code>y</code> from <code>dff</code>, it fails to find the keys. Also, <code>scatter</code> expects floats on the axes, while I have strings.</p>