Using Invisible Nodes

This is supposed to be an answer to this question.

It combines this example with the use of invisible nodes.

<!DOCTYPE html>
<meta charset="utf-8">

var width = 960, height = 500, colors = d3.scale.category10();

var n = 200, 
    k = 7,
    nodes = d3.range(n).map(function (num) { return {type: num % k, charge: -50} }),
    links = d3.range(1,n).map(function (a) { return {source: nodes[0], target: nodes[a]} });

nodes[0].fixed = true;
    nodes[0].x = 480;
    nodes[0].y = 250;

var invisibleNodes = d3.range(k).map(function () { return {charge: -1000} }),
    invisibleLinks = invisibleNodes.map(function (a) { return {source: 0, target: a} });

var force = d3.layout.force()
    .size([width, height])
    .nodes(nodes.concat(invisibleNodes))
    .links(invisibleLinks)
    .charge(function (d) { return d.charge })
    .on("tick", tick)
    .start();

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

svgLinks = svg.selectAll(".link").data(links)
  .enter().append("line")
    .attr("class", "link")

svgNodes = svg.selectAll(".node").data(nodes)
  .enter().append("circle")
    .attr("class", "node")
    .attr("r", 8)
    .style("fill", function (d) { return colors(d.type) })
    .call(force.drag);

function tick(e) {
  var k = .3 * e.alpha;

// Push nodes toward their designated focus.
  nodes.forEach(function(o, i) { 
    o.y += (invisibleNodes[o.type].y - o.y) * k;
    o.x += (invisibleNodes[o.type].x - o.x) * k;
  });

svgNodes
      .attr("cx", function(d) { return d.x })
      .attr("cy", function(d) { return d.y });

svgLinks
      .attr("x1", function(d) { return d.source.x })
      .attr("y1", function(d) { return d.source.y })
      .attr("x2", function(d) { return d.target.x })
      .attr("y2", function(d) { return d.target.y });
};

</script>

Modified http://d3js.org/d3.v3.min.js to a secure url

https://d3js.org/d3.v3.min.js