2. Create a line chart using d3

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

.line { fill: black; }

</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.scaleBand()
          .range([0, width])
          .padding(50);
var y = d3.scaleLinear()
          .range([height, 0]);

// append the svg object to the body of the page
// append a 'group' element to 'svg'
// moves the 'group' element to the top left margin
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 + ")");

console.log("x1996:", x(+'1996'));
          console.log("x2001:", x(+'2001'));
          console.log("x2014:", x(+'2014'));

//line function
var line = d3.line()
    .x(function(d) { console.log("x:", x(d.key));
    return x(d.key); }) // set the x values for the line generator
    .y(function(d) { console.log("y:", y(d.value.sumworth));return y(d.value.sumworth); }) // set the y values for the line generator
    .curve(d3.curveMonotoneX) // apply smoothing to the line

// get the billionaires data
d3.csv("billionaires.csv", function(error, data) {
  if (error) throw error;
  console.log(data);
  var worthMap = new Map();

data.forEach(function(d) {
    d.year = d.year;
    d['worth in billions'] = +d['worth in billions'];
   });

// data.forEach(function(d) {
   //   if (worthMap.has(d.year)) {
   //     sum = worthMap.get(d.year)
   //     sum = sum + (+d['worth in billions'])
   //     worthMap.set(d.year, sum);
   //   }else {
   //     worthMap.set(d.year,  +d['worth in billions']);
   //   }
   //  });
   //  console.log(worthMap);

// var arr = Array.from(worthMap.values());

// Scale the range of the data in the domains
  x.domain(nested_data.map(function(d) { return d.key; }));
  y.domain([0, d3.max(nested_data, function(d) { return +d.value.sumworth; })]);

// // 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", "15px")
        .attr("dy", ".75em")

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

// draw the line for the line chart
  svg.append("path")
      .datum(nested_data)
      .attr("fill", "none")
      .attr("stroke", "steelblue")
      .attr("stroke-linejoin", "round")
      .attr("stroke-linecap", "round")
      .attr("stroke-width", 1.5)
      .attr("d", line);

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 +","+(height/2)+")rotate(-90)")  // text is drawn off the screen top left, move down and out and rotate
            .text("Total 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)+")")  // text is drawn off the screen top left, move down and out and rotate
            .text("Year");

});

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

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