anchor.diff:          .attr("cy", function(d) { return d.y; })
           .attr("r", function(d) { return Math.sqrt(d.size) / 10 || 4.5; })
           .style("fill", color)
  -        .on("click", click)
  +        .on("click", toggle)
           .call(force.drag);
   
       // Exit any old nodes.
  
positive.code:   d3.json("visualisation-example-data.json", function(json) {
      root = json;
      update();
    });
  
    function update() {
  
negative.code:   }
  
    // Toggle children on click.
    function toggle(d) {
      if (d.children) {
        d._children = d.children;
        d.children = null;
  