working

<!DOCTYPE html>
<meta charset="utf-8">
<title>md-geometry</title>
<body>
<script src="//d3js.org/d3.v4.min.js"></script>
<script src="//d3js.org/topojson.v2.min.js"></script>
<script src="//d3js.org/d3-scale-chromatic.v1.min.js"></script>
<script>

var width = 960, height = 500;

var svg = d3.select('body').append('svg')
    .attr('width', width)
    .attr('height', height);

// For d3 representation of various stateplanes, see: d3-stateplane
// We'll set the scale and translate later, based on the data
var projection = d3.geoConicConformal()
    .parallels([40 + 40 / 60, 41 + 2 / 60])
    .rotate([74, -40 - 10 / 60]);

var path = d3.geoPath()
    .projection(projection);

var threshold = d3.scaleThreshold()
    .domain([1, 10, 50, 200, 800, 2000, 5000, 10000])
    .range(d3.schemeOrRd[9]);

d3.queue()
    .defer(d3.json, "https://raw.githubusercontent.com/umbcvis/classes/master/class-12/tracts.json")
    .defer(d3.json, "https://raw.githubusercontent.com/umbcvis/classes/master/class-12/population.json")
    .await(ready);

function ready(error, json, population) {
  if (error) throw error;

// Convert topojson to GeoJSON
  geojson = topojson.feature(json, json.objects.tracts);

// Set the projection's scale and translate based on the GeoJSON
  projection.fitSize([960, 500], geojson);

// Extract an array of features (one tract for each feature)
  tracts = geojson.features;

// Compute population density for each census tract
  tracts.forEach(function(tract, i) {
      var countyfips = tract.properties.COUNTYFP;
      var tractce = tract.properties.TRACTCE;
      var pop = +population.filter(function(d) { return (d[2] === countyfips) && (d[3] === tractce); })[0][0];
      var aland = tract.properties.ALAND / 2589975.2356; // area in square miles
      tract.properties.density = pop / aland;
  });

// Tracts
  svg.selectAll('path.tract')
      .data(tracts)
    .enter().append('path')
      .attr('d', path)
      .attr('class', 'tract')
      .style('fill', function(d) { return threshold(d.properties.density); })
      .style('stroke', '#000')
      .style('stroke-opacity', '0.15')

// Remove all counties except the county with FIPS code = 003
  svg.selectAll('path.tract')
      .filter(function(d) { return d.properties.COUNTYFP !== "003" })
      .remove();

// Get all tracts in the county with FIPS = '003'
  county = json.objects.tracts.geometries
      .filter(function(d) { return d.properties.COUNTYFP === '033'; });

// Plot the boundry of the county with FIPS = "003"
  svg.append("path")
       .datum(topojson.merge(json, county))
       .attr('class', 'county')
       .attr('d', path)
       .style('stroke', '#aaa')
       .style('stroke-width', '2px')
       .style('fill', 'none');

addLegend();
}

function addLegend() {

var formatNumber = d3.format("d");

var x = d3.scalePow().exponent('.15')
      .domain([1, 80000])
      .range([0, 300]);

var xAxis = d3.axisBottom(x)
      .tickSize(13)
      .tickValues(threshold.domain())
      .tickFormat(formatNumber)

var g = svg.append("g")
      .attr('transform', 'translate(100, 200)')
      .call(xAxis);

g.select(".domain")
      .remove();

g.selectAll("rect")
    .data(threshold.range().map(function(color) {
      var d = threshold.invertExtent(color);
      if (d[0] == null) d[0] = x.domain()[0];
      if (d[1] == null) d[1] = x.domain()[1];
      return d;
    }))
    .enter().insert("rect", ".tick")
      .attr("height", 8)
      .attr("x", function(d) { return x(d[0]); })
      .attr("width", function(d) { return x(d[1]) - x(d[0]); })
      .attr("fill", function(d) { return threshold(d[0]); });

g.append("text")
      .attr("fill", "#000")
      .attr("font-weight", "bold")
      .attr("text-anchor", "start")
      .attr("y", -6)
      .text("Population per square mile");
}
</script>

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

https://d3js.org/topojson.v2.min.js

https://d3js.org/d3-scale-chromatic.v1.min.js