Force Layout - Gravity + Negative Charge(Repulsion)

<html>
    <head>
        <title>D3 Particle System</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js"></script>
    </head>
    <body>
        <script>
            var w = 800;
            var h = 500;

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

var colorScale = d3.scale.category20();

var force = d3.layout.force()
                    .size([w, h])
                    .gravity(0.02)
                    .charge(-10)
                    .on("tick", tick);

var prevNode;
            svg.on("mousemove", function () {
                node = {x: d3.event.x, y: d3.event.y};
                node.px = prevNode != undefined ? prevNode.x : node.x;
                node.py = prevNode != undefined ? prevNode.y : node.y;
                prevNode = node;

svg.append("circle")
                        .data([node])
                        .attr("cx", node.x)
                        .attr("cy", node.y)
                        .attr("r", 5)
                        .attr("fill", colorScale(node.x + node.y))
                        .transition()
                        .delay(2000)
                        .attr("r", 0)
                        .each("end", function () {
                            force.nodes().shift();
                        })
                        .remove();

force.nodes().push(node);
                force.start();
            })
        </script>
    </body>
</html>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js