function d3_transition_each(callback) {
for (var j = 0, m = this.length; j < m; j++) {
for (vargroup = this[j], i = 0, n = group.length; i < n; i++) {
var node = group[i];
if (node) callback.call(node = node.node, node.__data__, i, j);
}
}
returnthis;
}