nodes and links

i wanna that... hmm nodes over links... understand?? please help me...

HTML

<script src="http://d3js.org/d3.v3.min.js"></script>
  </head>
<body><div id="graph"></div></body>

CSS

.link {
            stroke: #ccc;
            stroke-opacity: .6;
        }

        .node {
            stroke-width: 1.0px;
            cursor: pointer;
        }

        .nodetext {
            pointer-events: none;
            font: 10px sans-serif;
        }

        body {
            width: 100%;
            height: 100%;
            margin: none;
            padding: none;
        }

        #graph {
            width: 500px;
            height: 500px;
            border: 3px solid black;
            border-radius: 12px;
            margin: auto;
        }

JavaScript

function myGraph(el) {

        // Add and remove elements on the graph object
        this.addNode = function (id) {
            nodes.push({ "id": id });
            update();
        }

        this.removeNode = function (id) {
            var i = 0;
            var n = findNode(id);
            while (i < links.length) {
                if ((links[i]['source'] == n) || (links[i]['target'] == n)) links.splice(i, 1);
                else i++;
            }
            nodes.splice(findNodeIndex(id), 1);
            update();
        }

        this.addLink = function (source, target) {
            links.push({ "source": findNode(source), "target": findNode(target) });
            update();
        }

        var findNode = function (id) {
            for (var i in nodes) { if (nodes[i]["id"] === id) return nodes[i] };
        }

        var findNodeIndex = function (id) {
            for (var i in nodes) { if (nodes[i]["id"] === id) return i };
        }

        // set up the D3 svg in the specified element
        var w = $(el).innerWidth(),
            h = $(el).innerHeight();

        var svg = this.svg = d3.select(el).append("svg:svg")
            .attr("width", w)
            .attr("height", h);

        var force = d3.layout.force()
            .gravity(.05)
            .distance(100)
            .charge(-120)
            .size([w, h]);

        var nodes = force.nodes(),
            links = force.links();
        
        svg.append("g").attr("class", "links");
        svg.append("g").attr("class", "nodes");

        var update = function () {

            var link = svg.select(".links").selectAll(".link")
             .data(links, function (d) { return d.source.id + "-" + d.target.id; })

            link.enter().append("line")
             .attr("class", "link")

            link.exit().remove();

            var node = svg.select(".nodes").selectAll(".node")
                .data(nodes, function (d) { return d.id; })
           ...