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HTML 

              
                
<script src="https://d3js.org/d3.v4.js"></script>
<h1 class="heading" id='title'>US Gross Domestic Product (1947-2015)</h1>
<div id="graph">
    <svg class="chart"></svg>
</div>
!

CSS 

              
                .heading{
  margin-top: 50px;
  text-align: center;
  font-family: Courier;
}

#graph{
  margin-top: 25px;
  margin-left: auto;
  margin-right: auto;
  width: 910px;
  height:500px;
}

/* .chart{
  border: dotted 2px gray;
}  */
/*  .chart div {
    background-color: steelblue;  
    padding: 1px; 
    margin: 1px; 
    border: solid 1px blue;   
    fill: steelblue;    
  */
  
/*  .chart rect {
  border: solid 1px green;   */
/*   fill: gray; */
/* }     */

.x.axis path {
/*   display: none; */
}

.bar {
    fill: silver;
    cursor: pointer;
}

.bar:hover {
    fill: darkgray;
}

.d3-tip{
  text-align: center;
  line-height: 1;
  border: solid 1px gray;
  border-radius: 2px;
  font-family: Arial;
  font-size: 10px;
}

.d3-tip: after{
  box-sizing: border-box;
  display: inline;
  position: absolute;
  font-size: 10px;
}

.d3-tip.n: after{
  margin: -1px 0 0 0;
  top: 100%;
  left: 0;
}
!

JS 

              
                var url = "https://raw.githubusercontent.com/FreeCodeCamp/ProjectReferenceData/master/GDP-data.json";

//pull GDP data from json file into 'result' array
d3.json(url, function(error, data){     
  result = data.data;
  console.log(result);
  
  //set dimensions of graph
  var chartWidth = 910;
  var chartHeight = 500;  
  var margin = {
      top: 20,
      right: 30,
      bottom: 30,
      left: 60
    }
  var width = chartWidth - margin.right - margin.left;
  var height = chartHeight - margin.top - margin.bottom;
  //console.log(width);
  //console.log(height);
  
  //set width of each bar
  var barWidth = Math.ceil(width / result.length);
  //console.log(barWidth)
  
  //set range and domain for GDP value
  var y = d3.scaleLinear()
    .range([0, height])
    .domain([d3.max(result, function(v){return v[1];}),0])
  
  //set y-axis
  var yAxis = d3.axisLeft()
    .scale(y)
    .ticks(20);
  
  //set and convert start dates and end date for x-axis
  var startDate = new Date(result[0][0]);
  var endDate = new Date(result[result.length-1][0]);
  //console.log(startDate)
  //console.log(endDate)
  
  //set time along x-axis
  var x = d3.scaleLinear()
    .range([0, width])
    .domain([startDate, endDate]);
  
  //set x-axis
  var xAxis = d3.axisBottom(x)
    .tickFormat(d3.timeFormat("%Y"))
    .ticks(16);
  
  //when mouse over a bar, sets a tooltip with the GDP amount and exact year and month that bar represents.
  var tooltip = d3.select("#graph")
    .append("div")
    .attr("class","d3-tip")
    .attr("id","tooltip")
    .style("position","absolute")
    .style("margin-left","-50px")
    .style("margin-top","-40px")
    .style("padding","5px")
    .style("text-align","left")
    .style("background-color","white")
    .attr("id","tooltip")
    // .style("visibility","hidden")

  //sets chart
  var chart = d3.select(".chart")
    .attr("width", chartWidth)
    .attr("height", chartHeight)
    .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");
  
  //sets x- and y-values in the chart
  var bar = chart.selectAll("g")
    .data(result)
    .enter()
    .append("g")
    .attr("transform", function(d, i) {
      return "translate(" + barWidth * i + ",0)";
    })

  
  bar.append("rect")
    .attr("width", barWidth)
    .attr("height", function(v) {return height - y(v[1]);})
    .attr("y", function(v) {return y(v[1]);})
  	.style('stroke', 'lightgray')
    .attr("class", "bar")
    .on("mouseover", function(d)             { var format = d3.timeFormat("%B %Y"); tooltip.style("visibility","visible").html("Date: "+format(new Date(d[0]))+"<br/>"+"GDP: $"+(d[1]/1000).toFixed(2)+" trillion")
    .attr("data-date",v[0])                                 
                                              
    })
    .on("mousemove", function(){return tooltip.style("top", d3.event.pageY + "px").style("left",(d3.event.pageX)+"px");})

  // .on("mouseout", function(){ return tooltip.style("visibility","hidden");})

  chart.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")
    .call(xAxis)
    .attr("id","x-axis");

  chart.append("g")
    .call(yAxis)
    .attr("id","y-axis");
  
  chart.append("g")
    .append("text")
    .attr("x", -50)
    .attr("y", -10)
    .style("font-size", "10px")
    .style("font-family","Arial")
    .text("US$ billions")
  
  // d3.selectAll("rect").style("fill", function(d, i) {
  //   return i % 2 ? "darkgray" : "silver";
  // });
  
});
!
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