Updating links on a force directed graph from dynamic json data

I am new to D3 and working on a force directed graph where the json data is dynamic. I am able to change the force graph upon receiving new data but that happens with a springing effect. The code that creates my force graph is :

<div class="graph"></div>
<script>
var w = 660,
    h = 700,
    r = 10;
var vis = d3.select(".graph")
    .append("svg:svg")
    .attr("width", w)
    .attr("height", h)
    .attr("pointer-events", "all")
    .append('svg:g')
    .call(d3.behavior.zoom().on("zoom", redraw))
    .append('svg:g');
vis.append('svg:rect')
    .attr('width', w)
    .attr('height', h)
    .attr('fill', 'rgba(1,1,1,0)');
function redraw() {
    console.log("here", d3.event.translate, d3.event.scale);
    vis.attr("transform", "translate(" + d3.event.translate + ")" +
                          " scale(" + d3.event.scale + ")");
};  

var force = d3.layout.force()
    .gravity(.05)
    .charge(-200)
    .linkDistance( 260 )
    .size([w, h]);

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

d3.json(graph, function(json) {

    var nodeList = json.nodes;
    var link = vis.selectAll("line")
        .data(json.links)
       .enter()
        .append("line")
        .attr("stroke-opacity", function(d) {
            if(d.label == 'is a') {
                return '0.8';
            } else {
                return '0.2';
            };
        })
        .attr("stroke-width", function(d) {
            if(d.value !== null) {
                return d.value;
            } else {
                return 2;
            };
        })
        .style("stroke", function(d) {
            if(d.color !== null) {
                return d.color;
            };
        })
        .on("mouseover", function() {
            d3.select(this)
                .style("stroke", "#999999")
                .attr("stroke-opacity", "1.0");
        })
        .on("mouseout", function() {
            d3.select(this)