4. Create a scatter plot - trivariate using d3

<!DOCTYPE html>
<meta charset="utf-8">
<style> /* set the CSS */

</style>
<body>

<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

// set the dimensions and margins of the graph
var margin = {top: 20, right: 20, bottom: 50, left: 50},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

// set the ranges
var x = d3.scaleLinear()
          .range([0, width]);
var y = d3.scaleLinear()
          .range([height, 0]);

var cValue = function(d) {
  return d["gender"];},
    color = d3.scaleOrdinal(d3.schemeCategory20);

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform",
          "translate(" + margin.left + "," + margin.top + ")");

// get the billionaires data
d3.csv("billionaires.csv", function(error, data) {
  if (error) throw error;
  data.sort(function(x, y){
   return d3.ascending(x.age, y.age);
  })
  console.log(data);

// data.forEach(function(d) {
  //   console.log(d["worth in billions"]);
  //  });
  // Scale the range of the data in the domains
  x.domain([d3.min(data, function(d) { return +d['age']; }), d3.max(data, function(d) { return +d['age']; })]);
  y.domain([0, d3.max(data, function(d) { return +d['worth in billions']; })]);

svg.selectAll(".dot")
      .data(data)
    .enter().append("circle")
      .attr("class", "dot")
      .attr("r", 3.5)
      .attr("cx", function(d) { return x(d['age']); })
      .attr("cy", function(d) { return y(d["worth in billions"]); })
      .style("fill", function(d) { return color(cValue(d))});
  // add the x Axis
  svg.append("g")
      .attr("transform", "translate(0," + height + ")")
      .call(d3.axisBottom(x))
		  .selectAll("text")
      .style("text-anchor", "end")
        .attr("dx", ".8em")
      .style("font-size", "10px")
        .attr("dy", ".75em")

// add the y Axis
  svg.append("g")
    .call(d3.axisLeft(y));

svg.append("text")
            .attr("text-anchor", "middle")  // this makes it easy to centre the text as the transform is applied to the anchor
            .attr("transform", "translate("+ -margin.right*1.1 +","+(height/2)+")rotate(-90)")  // text is drawn off the screen top left, move down and out and rotate
            .text("Worth in Billions");

svg.append("text")
            .attr("text-anchor", "middle")  // this makes it easy to centre the text as the transform is applied to the anchor
            .attr("transform", "translate("+ (width/2) +","+(height+margin.bottom/2)+")")
              // text is drawn off the screen top left, move down and out and rotate
            .text("Age");

//Legend
// draw legend
 var legend = svg.selectAll(".legend")
     .data(color.domain())
     .enter().append("g")
     .attr("class", "legend")
     .attr("transform", function(d, i) { return "translate(0," + i * 20 + ")"; });

// draw legend colored rectangles
legend.append("rect")
    .attr("x", width - 18)
    .attr("width", 18)
    .attr("height", 18)
    .style("fill", color);

// draw legend text
legend.append("text")
    .attr("x", width - 24)
    .attr("y", 9)
    .attr("dy", ".35em")
    .style("text-anchor", "end")
    .text(function(d) {
      if(d == ""){
        return "undefined"
      }
      return d;
    })
});

</script>
<p><a href ="https://bl.ocks.org/elt12njo/76b484f5187c7ecfc83070dd81897327/6741bd95237f3ed45867a843e8e5403b90386639">Reference</a>	
</p>
</body>

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