code: public void setVertexOrder(Comparator<String> comparator)
  	{
  	    if (vertex_ordered_list == null)
  	        vertex_ordered_list = new ArrayList<String>(getGraph().getVertices());
  	    Collections.sort(vertex_ordered_list, comparator);
  	}
docs: Sets the order of the vertices in the layout according to the ordering
  specified by {@code comparator}.