title: d3.js transition end event
body: <p>I am applying a transition to a group of nodes returned by <code>selectAll()</code>. I thought the end event would fire after all transitions finished, but <code>each("end",function)</code> gets called at the end of each transition. </p>
  
  <p>So is there any way to set a callback that will be called after transitions on all selected  node finishes ?</p>
  
  <p>Should I use <code>call</code> for this? but I don't see it used as end callback anywhere in documentation.</p>
  
  <p>also I can run a counter inside <code>each</code> callback. but is there any way to know how many nodes are still pending to finish transition ? or index of current node in group of selected nodes ?</p>
  
  <p>I've two select() calls in chain like <code>selectAll('.partition').selectAll('.subpartition')</code>
  so index argument passed to each callback will rotated n times.</p>