title: Binding data in d3.js
body: <p>In this code: <a href="http://bl.ocks.org/mbostock/3886208" rel="nofollow noreferrer">http://bl.ocks.org/mbostock/3886208</a></p>
  
  <p>What is the function of the third (data) in </p>
  
  <pre><code>.data(stack.keys(data.columns.slice(1))(data))
  </code></pre>