Loading json files

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

.node {
  cursor: pointer;
}

.node:hover {
  stroke: #000;
  stroke-width: 1.5px;
}

.node--leaf {
  fill: white;
}

.label {
  font: 11px "Helvetica Neue", Helvetica, Arial, sans-serif;
  text-anchor: middle;
  text-shadow: 0 1px 0 #fff, 1px 0 0 #fff, -1px 0 0 #fff, 0 -1px 0 #fff;
}

.label,
.node--root,
.node--leaf {
  pointer-events: none;
}

</style>
<body>
<script src="https://d3js.org/d3.v3.min.js"></script>
<script>

var margin = 20,
    diameter = 960;

var color = d3.scale.linear()
    .domain([-1, 5])
    .range(["hsl(152,80%,80%)", "hsl(228,30%,40%)"])
    .interpolate(d3.interpolateHcl);

var pack = d3.layout.pack()
    .padding(2)
    .size([diameter - margin, diameter - margin])
    .value(function(d) { return d.size; })

///////////////////////
/* buttons */
var buttonData = ["flare.json", "flare1.json", "flare2.json"];
var buttonDiv = d3.select("body").append("svg")
    .attr("width", diameter)
    .attr("height", 50);
var buttons = buttonDiv.selectAll(".updateButton")
  .data(buttonData)
   .enter()
  .append('g')
  .attr("class", "updateButton")
  .on("click", function(d, i) {
    callJson(d);   
  });
buttons.append("rect")
  .attr("x", function(d, i) { return (i * 100) + 300; })
    .attr("width", 98)
    .attr("height", 25)
    .attr("ry", 5)
    .style("stroke", "#787878")
    .style("fill", "steelblue");
buttons.append("text")
    .attr("x", function(d, i) { return (i * 100) + (100 / 2) + 290; }) 
    .attr("y", 12)
    .attr("dy", "0.35em")
    .style("text-anchor", "middle")
    .style("font-size", "15px")
    .text(function(d) { return d; });
 ///////////////////////

var svg = d3.select("body").append("svg")
    .attr("width", diameter)
    .attr("height", diameter)
  .append("g")
    .attr("transform", "translate(" + diameter / 2 + "," + diameter / 2 + ")");

var callJson = function (json) {
  d3.json(json, function(error, root) {
    if (error) return console.error(error);

svg.selectAll("circle").remove();
    svg.selectAll("text").remove();

var focus = root,
        nodes = pack.nodes(root),
        view;

var node = svg.selectAll("circle,text");

d3.select("body")
        .style("background", color(-1))
        .on("click", function() { zoom(root); });

zoomTo([root.x, root.y, root.r * 2 + margin]);

function zoom(d) {
      var focus0 = focus; focus = d;

var transition = d3.transition()
          .duration(d3.event.altKey ? 7500 : 750)
          .tween("zoom", function(d) {
            var i = d3.interpolateZoom(view, [focus.x, focus.y, focus.r * 2 + margin]);
            return function(t) { zoomTo(i(t)); };
          });

transition.selectAll("text")
        .filter(function(d) { return d.parent === focus || this.style.display === "inline"; })
          .style("fill-opacity", function(d) { return d.parent === focus ? 1 : 0; })
          .each("start", function(d) { if (d.parent === focus) this.style.display = "inline"; })
          .each("end", function(d) { if (d.parent !== focus) this.style.display = "none"; });
    }

callJson("flare.json");

d3.select(self.frameElement).style("height", diameter + "px");

</script>

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

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