gistfile1.js

// graph nodes
var a = {label:"a"}, b = {label:"b"}, c = {label:"c"},
	d = {label:"d"}, e = {label:"e"}, f = {label:"f"},
	g = {label:"g"}, h = {label:"h"}, i = {label:"i"};

// setup dependency hierarchy (directed graph node edges)
a.children = [b,c,d,e];
c.children = [e,f];
d.children = [e,c];
h.children = [a,i,g];
i.children = [a];

// a -> b
// a -> c
// a -> d
// a -> e
// c -> e
// c -> f
// d -> e
// d -> c
// h -> a
// h -> i
// h -> g
// i -> a


// *************************************************


var nodes = [];

// depth-first graph nodes traversal
[a,b,c,d,e,f,g,h,i]
.forEach(function visit(node) {
	// adapted from: http://en.wikipedia.org/wiki/Topological_sorting#Algorithms
	if (node.marked) throw "Cycle!";
	if (!node.visited) {
		node.marked = true;
		if (node.children) {
			node.children.forEach(function eacher(n){
				n.parents = n.parents || [];
				n.parents.push(node);
				visit(n);
			});
		}
		node.visited = true;
		delete node.marked;
		delete node.children;
		nodes.unshift(node); // unshift reverses ordering
	}
});

// calculate depths
nodes.forEach(function eacher(node){
	node.depth = 0;
	delete node.visited;
	if (node.parents) {
		node.parents.forEach(function eacher(n){
			node.depth = Math.max(n.depth + 1,node.depth);
		});
		delete node.parents;
	}
});

// sort by depth
nodes.sort(function sorter(a,b){
	return b.depth - a.depth;
});

console.log(nodes);
// [ { label: 'f', depth: 5 },
//   { label: 'e', depth: 5 },
//   { label: 'c', depth: 4 },
//   { label: 'd', depth: 3 },
//   { label: 'b', depth: 3 },
//   { label: 'a', depth: 2 },
//   { label: 'i', depth: 1 },
//   { label: 'g', depth: 1 },
//   { label: 'h', depth: 0 } ]


// *************************************************


// group by depth
nodes = nodes.slice(1).reduce(function reducer(nodes,node){
	var prev = nodes[nodes.length-1];
	if (Array.isArray(prev) && prev[0].depth === node.depth) {
		prev.push(node);
	}
	else if (prev.depth === node.depth) {
		nodes[nodes.length-1] = [prev,node];
	}
	else {
		nodes.push(node);
	}

	return nodes;
},[nodes[0]]);

console.log(nodes);
// [ [ { label: 'f', depth: 5 }, { label: 'e', depth: 5 } ],
//   { label: 'c', depth: 4 },
//   [ { label: 'd', depth: 3 }, { label: 'b', depth: 3 } ],
//   { label: 'a', depth: 2 },
//   [ { label: 'i', depth: 1 }, { label: 'g', depth: 1 } ],
//   { label: 'h', depth: 0 } ]