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              <p class="title">Stacking &amp; Animating Formula 1 Circuits in D3</p>
  <a href="https://codepen.io/jwasilgeo/pen/vaewyo" target="_blank">Part 1</a>
  <span style="font-weight: bold; background-color: yellow;">Part 2</span>
  <a href="https://codepen.io/jwasilgeo/pen/ajKEJQ" target="_blank">Part 3</a>

<svg viewBox="0 0 500 500"></svg>

<p class="emojis">&#x1F3CE; &#x1F3CE; &#x1F3CE; &#x1F3C1; &#x1F3C6; &#x1f60e;</p>

<p>Each Formula 1 circuit is rendered using its own D3 map projection that is rotated to that circuit's location on Earth. They appear to be stacked on top of each other because they are placed in the same SVG element.</p>

<script src="https://d3js.org/d3.v5.min.js"></script>
<script src="https://unpkg.com/topojson-client@3.0.0/dist/topojson-client.min.js"></script>
              body {
  margin: 10px auto;
  text-align: center;
  font-size: 0.95em;

svg {
  stroke-linejoin: round;
  stroke-linecap: round;
  max-width: 78vmin;

svg .circuit-path {
  stroke-width: 2px;
  stroke: steelblue;
  fill: none;
  opacity: 0.4;
  cursor: pointer;
  transform-origin: center;
  transition: stroke-width 0.25s, stroke 0.25s, opacity 0.25s;

svg .circuit-path:hover {
  stroke-width: 5.5px;
  stroke: red;
  opacity: 0.6;

p {
  margin: 0 auto 5px auto;
  max-width: 80%;

p.title {
  font-weight: bold;

p.emojis {
  font-size: 1.5em;

  .then(function(f1CircuitsTopoJson) {
    // topojson to geojson conversion
    var f1CircuitsGeoJson = topojson.feature(

    var svg = d3.select("svg");
    var dimensions = svg.attr("viewBox").split(" ");
    var width = dimensions[2];
    var height = dimensions[3];

    // create a child <path> element in the <svg> for every geojson circuit feature
    // and draw geojson features by converting (projecting) from geospatial coordinates
    // to svg space using a unique projection and geoPathGenerator for each circuit
      .attr("d", function(geoJsonFeature) {
        // as each geojson feature is being drawn to an svg path,
        // use a unique projection that is rotated to the centroid of the geojson feature
        // which will make all the features "stacked" on top of each other in svg coordinate space
        var featureCentroid = d3.geoCentroid(geoJsonFeature);
        var featureCentroidRotation = [

        var projection = d3
          // half of the parent <svg> width and height
          .translate([width / 2, height / 2])
          // this scale seems to work well for all the circuits
          .scale(2400 * width)
          // the projection rotation is what is most important for stacking features and
          // it is calculated by finding the centroid coordinates of the geojson circuit

        // the geoPathGenerator is responsible for generating the "d" attribute that a <path> element needs
        var geoPathGenerator = d3.geoPath(projection);

        return geoPathGenerator(geoJsonFeature);
      .classed("circuit-path", true)
      .style("transform", "scale(0.01)")
      // give each new <path> element a <title> child element to act as a tooltip
      .text(function(d) {
        return d.properties.Name;

    // transition in all the circuits' <path> elements
    // this will also start the endless yoyo of in/out transitions
    transformCircuitsYoyo("none", d3.easeBackOut);

    function transformCircuitsYoyo(transformValue, easing) {
        .each(function(d, idx, nodeList) {
            .delay(100 * idx)
            .style("transform", transformValue)
            .on("end", function() {
              while (idx < nodeList.length - 1) {

              var nextTransformValue =
                transformValue === "scale(0.01)" ? "none" : "scale(0.01)";

              var nextEasing =
                easing === d3.easeBackIn ? d3.easeBackOut : d3.easeBackIn;

              var nextDelay = transformValue === "scale(0.01)" ? 250 : 2500;

              setTimeout(function() {
                transformCircuitsYoyo(nextTransformValue, nextEasing);
              }, nextDelay);

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