D3-Force: Split Categorical

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

d3.csv("data.csv", function(error, data) {

////////////////////////
  ////////////////////////
  // Everything unique to this bl.ock is in this function:
  function categoricalSplit() {
    // Create a scale to translate from categorical (string) data value
    // to a point on the screen (effectively an invisible axis)
    var catScale = d3.scalePoint()
        .domain(data.map(function(d) { return d['continent']; }))
        .range([0, width])
        .padding(0.5); // give some space at the outer edges

// Add some labels to show whats happening with the split groups
    var labels = svg.selectAll("text")
        .data(catScale.domain()) // heh, scales take care of the unique, so grab from there
        .enter().append("text")
        .attr("class", "label")
        .text(function(d) { return d; })
        .attr("fill", "#DDD")
        .attr("text-anchor", "middle")
        .attr("x", function(d) { return catScale(d); })
        .attr("y", height / 2.0 - 100);

var xCatForce = d3.forceX(function(d) {
      return catScale(d['continent']);
    });

// Interaction with button
    var splitState = false;
    document.getElementById("split-button").onclick = function() {
      if(!splitState) {
        // push the nodes towards respective spots
        simulation.force("x", xCatForce);
        // emphasize labels
        labels.attr("fill", "#000");
      } else {
        // reset
        simulation.force("x", centerXForce);
        labels.attr("fill", "#DDD");
      }

// Toggle state
      splitState = !splitState;

// NOTE: Very important to call both alphaTarget AND restart in conjunction
      // Restart by itself will reset alpha (cooling of simulation)
      // but won't reset the velocities of the nodes (inertia)
      simulation.alpha(1).restart();
    };
  }

////////////////////////
  ////////////////////////
  ////////////////////////
  // The rest of this file is from:
  //  https://bl.ocks.org/cjhin/4c990c57b9b05e58d56b396751f9747d

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

// "Electric repulsive charge", prevents overlap of nodes
  var chargeForce = d3.forceManyBody()

// Keep nodes centered on screen
  var centerXForce = d3.forceX(width / 2);
  var centerYForce = d3.forceY(height / 2);

// Apply default forces to simulation
  var simulation = d3.forceSimulation()
      .force("charge", chargeForce)
      .force("x", centerXForce)
      .force("y", centerYForce);

var node = svg.selectAll("circle")
      .data(data)
      .enter().append("circle")
      .attr("r", 10)
      .attr("fill", "#777");

// Add the nodes to the simulation, and specify how to draw
  simulation.nodes(data)
      .on("tick", function() {
        // The d3 force simulation updates the x & y coordinates
        // of each node every tick/frame, based on the various active forces.
        // It is up to us to translate these coordinates to the screen.
        node.attr("cx", function(d) { return d.x; })
            .attr("cy", function(d) { return d.y; });
      });

// Call the function unique to this block
  categoricalSplit();
});
</script>

#split-button{
    position: absolute;
    bottom: 10px;
    right: 10px;
    padding: 10px 20px;
    font-size: 2em;
    text-align: center;
    background: #FFF;
    border-radius: 5px;
    border: 1px solid #DDD;
  }

#split-button:hover {
    background: #CCC;
    cursor: pointer;
  }
</style>

<body>
  <div id="split-button">Toggle Split</div>
  <svg width="960" height="500"></svg>
</body>

https://d3js.org/d3.v4.min.js