title: D3.js selection.data() optional parameters
body: <p>Per D3 v4 <a href="http://github.com/d3/d3-selection/blob/master/README.md#selection_%E2%80%8C%E2%80%8Bdata" rel="nofollow">documentation</a>, function syntax for <code>data()</code> is given as <code>selection.data([data[, key]])</code>.</p>
  
  <p>This suggests to me <code>data</code> AND <code>key</code> are optional parameters for <code>data()</code>.  But wouldn't <code>data</code> always be a required parameter?  I know <code>key</code> is optional and defaults to the array index if not specified - but why would the <code>data</code> parameter ever be optional, what is the point of binding nothing to DOM elements?</p>