county names May 5

<!DOCTYPE html>
<meta charset="utf-8">
<style>
path {
  fill: #555555;
  stroke: #aaaaaa;
}
body {
  position: absolute;
  margin: 0px;
  font: 16px sans-serif;
}
.info {
  color: #000;
  position: absolute;
  top: 450px;
  left: 800px;
}
.tooltip {
  position: absolute;
  visibility: visible;
  background-color: #aaa;
  padding: 5px;
}
/* This style is used when dragging the dot */
.active {
  stroke: #000;
  stroke-width: 2px;
}
path {
  fill: #555555;
  stroke: #aaaaaa;
}

svg {
  background-color: #4682b4;
}

</style>

<body>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src ="https://d3js.org/topojson.v2.min.js"></script>
<script src="https://d3js.org/d3-scale-chromatic.v1.min.js"></script>
<script>
var width = 960, height = 500;

var data;  // declare a global variable

var svg = d3.select('body').append('svg')
    .attr('width', width)
    .attr('height', height)
var threshold = d3.scaleThreshold()
    .domain([1, 10, 20, 200, 800, 2000, 5000, 10000])
    .range(d3.schemeOrRd[9]);

d3.queue()
    .defer(d3.json, 'https://umbcvis.github.io/classes/class-12/tracts.json')
    .defer(d3.json, 'https://umbcvis.github.io/classes/class-12/population.json')
    .await(ready);

// Note: scale and translate will be determined by the data
var projection = d3.geoConicConformal()
    .parallels([38 + 18 / 60, 39 + 27 / 60])
    .rotate([77, -37 -40 / 60]);

var path = d3.geoPath()
    .projection(projection);
function ready(error, json, population) {
    if (error) throw error;

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

// 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.forEach(function(tract) {
        var countyfips = tract.properties.COUNTYFP;
        var tractce = tract.properties.TRACTCE;
        pop =  population.filter(function(d) { return (d[2] === countyfips)
                                                   && (d[3] === tractce); });
        pop = +pop[0][0];
        var aland = tract.properties.ALAND / 2589975.2356;

//area in square miles
        tract.properties.density = pop / aland;
    });
    svg.selectAll('path.tract')
        .data(tracts)
      .enter()
        .append('path')
        .attr('class', 'tract')
        .attr('d', path)
        .style('fill', function (d) {return threshold(d.properties.density); })
        .style('stroke', '#000')

// Draw all counties in MD
    fips = geojson.features.map(function(d) { return d.properties.COUNTYFP; });
    uniqueFips = d3.set(fips).values();
    counties = uniqueFips.map(function(fips) {
      return json.objects.tracts.geometries
          .filter(function(d) { return d.properties.COUNTYFP === fips; });
    });
    // Construct county boundary from tracts, and add state & county FIPS
    counties = counties.map(function(countyTracts) {
      var county = topojson.merge(json, countyTracts);
      county.statefp = countyTracts[0].properties.STATEFP;
      county.countyfp = countyTracts[0].properties.COUNTYFP;
      return county;
    })
    svg.selectAll("path.county")
        .data(counties)
      .enter()
        .append('path')
        .attr('class', 'county')
        .attr('d', path)
        .style('stroke', 'white')
        .style('stroke-width', '2px')
        .style('fill', 'none');

// 1. NEW: Define an array with Rockville MD longitude & latitude
    var Rockville = [-77.1528, 39.0840];
    // 2. NEW: Add an HTML <div> element that will function as the tooltip
    var tooltip = d3.select('body').append('div')
        .attr('class', 'tooltip')
        .text('Hello, world!')
    // 3. NEW: Add a draggable circle to the map
  // See: https://bl.ocks.org/mbostock/22994cc97fefaeede0d861e6815a847e)
    var layer2 = svg.append("g");
  layer2.append("circle")
      .attr("class", "Rockville")
      .attr("cx", projection(Rockville)[0])
      .attr("cy", projection(Rockville)[1])
      .attr("r", 10)
      .style("fill", "yellow")  // Make the dot yellow

.call(d3.drag()           // Add the drag behavior
        .on("start", dragstarted)
        .on("drag", dragged)
        .on("end", dragended));

function dragstarted(d) {
    // Highlight the circle that's being dragged
  d3.select(this).raise().classed("active", true);
    // Make the tooltip visible during drag events
  tooltip.style("visibility", "visible");
  }
function dragged(d) {
  // Reposition the circle to follow the mouse during drag
  d3.select(this).attr("cx", d3.event.x).attr("cy", d3.event.y);
 // Reposition the tooltip too
  tooltip.style("left", (d3.event.x - 30) + "px").style("top", (d3.event.y - 50) + "px");

// Add the name of the County to the tooltip
  d3.selectAll('path.county')
      .filter(function(d) { return d3.geoContains(d, projection.invert([d3.event.x, d3.event.y])) })
      .each(function(d) {
        console.log('https://github.com/MTClass/Project/blob/master/CountieListFIPS.csv', d);
        tooltip.html(d.statefp + d.countyfp);
      })
}
function dragended(d) {
  // Eliminate the highlight when dragging ends
  d3.select(this).classed("active", false);
  // Hide the tooltip
  tooltip.style("visibility", "hidden");
}
}
 //Add legend

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/d3-scale-chromatic.v1.min.js