uses the d3 event system to dispatch layout events such as:
o "start" (start layout process)
o "tick" (after each layout iteration)
o "end" (layout converged and complete).
uses the d3 timer to queue layout iterations.
sets up d3.behavior.drag to drag nodes
o use node.call(<the returned instance of Layout>.drag) to make nodes draggable
returns an instance of the cola.Layout itself with which the user
can interact directly.
provides an interface for use with d3:
node.call(<the returned instance of Layout>.drag)to make nodes draggable returns an instance of the cola.Layout itself with which the user can interact directly.