D3 Examples - Radial Force

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<!DOCTYPE html>
<meta charset="utf-8">
<svg width="960" height="500" viewBox="-480 -250 960 500">
  <circle r="100" stroke="brown" stroke-opacity="0.5" fill="none"></circle>
  <circle r="200" stroke="steelblue" stroke-opacity="0.5" fill="none"></circle>
</svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>
​
var nodes = [].concat(
  d3.range(80).map(function() { return {type: "a"}; }),
  d3.range(160).map(function() { return {type: "b"}; })
);
​
var node = d3.select("svg")
  .append("g")
  .selectAll("circle")
  .data(nodes)
  .enter().append("circle")
    .attr("r", 2.5)
    .attr("fill", function(d) { return d.type === "a" ? "brown" : "steelblue"; })
​
var simulation = d3.forceSimulation(nodes)
    .force("charge", d3.forceCollide().radius(5))
    .force("r", forceRadial(function(d) { return d.type === "a" ? 100 : 200; }))
    .on("tick", ticked);
​
function ticked() {
  node
      .attr("cx", function(d) { return d.x; })
      .attr("cy", function(d) { return d.y; });
}
​
function forceRadial(radius, x, y) {
  var nodes,
      strength = constant(0.1),
      strengths,
      radiuses;
​
  if (typeof radius !== "function") radius = constant(+radius);
  if (x == null) x = 0;
  if (y == null) y = 0;
​
  function force(alpha) {
    for (var i = 0, n = nodes.length; i < n; ++i) {
      var node = nodes[i],
          dx = node.x - x || 1e-6,
          dy = node.y - y || 1e-6,
          r = Math.sqrt(dx * dx + dy * dy),
          k = (radiuses[i] - r) * strengths[i] * alpha / r;
      node.vx += dx * k;
      node.vy += dy * k;
    }
  }
​
  function initialize() {
    if (!nodes) return;
    var i, n = nodes.length;
    strengths = new Array(n);
    radiuses = new Array(n);
    for (i = 0; i < n; ++i) {
      radiuses[i] = +radius(nodes[i], i, nodes);
      strengths[i] = isNaN(radiuses[i]) ? 0 : +strength(nodes[i], i, nodes);
    }
  }
​
  force.initialize = function(_) {
    nodes = _, initialize();
  };
​
  force.strength = function(_) {
    return arguments.length ? (strength = typeof _ === "function" ? _ : constant(+_), initialize(), force) : strength;
  };
​
  force.radius = function(_) {
    return arguments.length ? (radius = typeof _ === "function" ? _ : constant(+_), initialize(), force) : radius;
  };
​
  force.x = function(_) {
    return arguments.length ? (x = +_, force) : x;
  };
​
  force.y = function(_) {
    return arguments.length ? (y = +_, force) : y;
  };
​
  return force;
}
​
function constant(x) {
  return function() {
    return x;
  };
}
​
</script>
​
    