title: dc.js generate a pieChart with top 5 values
body: <p>I'm visualizing a drug database of 200 entries. I've created a piechart using DC.js to show the frequency of each drug. The pie chart is cluttered with drugs that appear once. </p>
  
  <p>I want to adjust this pie chart so that it visualizes the top 5 most used drugs i.e. the top 5 biggest slices of the pie chart.</p>
  
  <p>Is there a way to use the .group() or .reduceSum() methods so only the biggest pie chart slices are shown?</p>