1 d3_selectionPrototype.transition = function() {

     2   var subgroups = [],

     3       subgroup,

     4       node;

     5 

     6   for (var j = -1, m = this.length; ++j < m;) {

     7     subgroups.push(subgroup = []);

     8     for (var group = this[j], i = -1, n = group.length; ++i < n;) {

     9       subgroup.push((node = group[i]) ? {node: node, delay: 0, duration: 250} : null);

    10     }

    11   }

    12 

    13   return d3_transition(subgroups, d3_transitionInheritId || ++d3_transitionId, Date.now());

    14 };