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HTML

              
                <div id='header'>
  <h1>U.S. Education Attainment</h1>
  <h2>
    Percentage of adults age 25 and older with a bachelor's degree or higher (2010-2014)
    <p class="hint">Mouseover or click a color in the Legend to view all counties of that color.</p>
  </h2>
</div>
<div id='svg-container'>
  <svg id='graph' width='1000' height='600'></svg>
  
</div>
              
            
!

CSS

              
                body{
  position: relative;
  font-family: 'Arial';
}
#header {
  position: relative;
  display: inline-block;
  text-align: center;
  left: 200px;
}
h1,
h2{
  display: block;
}
h1{
  font-size: 30px;
}
h2{
  font-size: 16px;
  font-weight: normal;
}


.hint {
  opacity: 0.6;
}

#svg-container {
  margin:0 50px;
  padding:0;
}

svg {
  display: block;
}
.county {
  transition: 1s;
  transform-origin: 50% 50%;
  opacity: 0.1;
}
.view{
  opacity: 1;
}

.states{
  transition: 1s;
  fill: none;
  transform-origin: 50% 50%;
  stroke: white;
}
.zoom {
  transform: scale(1.02);
}

.tooltip {
  position: absolute;
  background-color: rgba(255,255,255,0.9);
  text-align: center;
  color: black;
  pointer-events: none;
  border-radius: 5px;
  padding: 2px;
  p{
    display: block;
    margin: 0;
    padding: 0 5px;
    font-size: 12px;
  }
}

#legend > rect {
  transition: 500ms;
}
              
            
!

JS

              
                const GEO_DATA='https://raw.githubusercontent.com/no-stack-dub-sack/testable-projects-fcc/master/src/data/choropleth_map/counties.json';
const EDUCATION_DATA='https://raw.githubusercontent.com/no-stack-dub-sack/testable-projects-fcc/master/src/data/choropleth_map/for_user_education.json';

const data={};
const themeHue=220;
var viewCategory=null;
//use promises to handle async fetch
function get(url){
  return new Promise(function(resolve,reject){
    var req=new XMLHttpRequest();
    req.open('GET',url);
    req.onload=function(){
      if(req.status==200){
        resolve(JSON.parse(req.response));
      }
      else {
        reject(Error(req.statusText));
      }
    };
    //handle network errors
    req.onerror=function(){
      reject(Error("Network Error"));
    }
    //make the request
    req.send();
  });
}
//run the function with promise chain
get(GEO_DATA)
  .then(response=>data.geo=response)
  .then(()=>get(EDUCATION_DATA))
  .then(response=>data.edu=response)
  .then(()=>build());

//begin drawing map
function build(){
  //variables,scales,functions
  const eduMin=d3.min(data.edu,(d)=>d.bachelorsOrHigher);
  const eduMax=d3.max(data.edu,(d)=>d.bachelorsOrHigher);
  const dataSteps=8;
  const lightMin=100;
  const lightMax=20;
  const lightStep=(lightMax-lightMin)/(dataSteps+1);
  const colorScale=d3.scaleThreshold()
    .domain(d3.range(eduMin,eduMax,(eduMax-eduMin)/dataSteps))
    .range(d3.range(lightMin,lightMax,lightStep));
  const colorHSL=(hue,light)=>{
    return 'hsl('+hue+',80%,'+light+'%)'
  }
  //identifiers
  let svg = d3.select('#graph');
  //debug
  console.log(data);
  //draw legend
  const legendX=605;
  const legendY=15;
  const legendH=10;
  const legendW=30;
  let legendScale=d3.scaleLinear()
    .domain([eduMin,eduMax])
    .range([0,legendW*colorScale.domain().length]);
  const legendAxis=d3.axisBottom(legendScale)
    .tickValues(colorScale.domain().concat(eduMax))
    .tickFormat((d)=>Math.round(d)+'%')
    .tickSize(10);
  const legend=svg.append('g')
    .attr('id','legend')
    .attr('transform','translate('+legendX+','+legendY+')');
  legend.selectAll('rect')
    .data(colorScale.domain())
    .enter()
    .append('rect')
    .attr('id',d=>'interval-'+Math.round(colorScale(d)))
    .attr('x',(d,i)=>i*legendW)
    .attr('y',legendY)
    .attr('width',legendW)
    .attr('height',legendH)
    .style('cursor','pointer')
    .style('fill',(d)=>colorHSL(themeHue,colorScale(d)))
    //click to freeze the map and view a specific category
    .on('click',(d)=>{
      if(viewCategory==d3.select('#interval-'+Math.round(colorScale(d))).attr('id').slice(9)) viewCategory=null;
      else if (!viewCategory) viewCategory = Math.round(colorScale(d));
    })
    .on('mouseenter',function(d){
      //freeze legend animations when viewing a category
      if(!viewCategory){
        d3.select(this)
          .attr('y',legendY-10)
          .attr('height',legendH+10);
        d3.selectAll('.states,.county-'+Math.round(colorScale(d)))
          .classed('zoom',true);
        d3.selectAll('.county:not(.county-'+Math.round(colorScale(d))+')')
          .classed('view',false);
      }
    })
    .on('mouseleave',function(d){
      //freeze legend animations when viewing a category
      if(!viewCategory){
        d3.select(this)
          .attr('y',legendY)
          .attr('height',legendH);
        d3.selectAll('.states,.county-'+Math.round(colorScale(d)))
          .classed('zoom',false);
        d3.selectAll('.county:not(.county-'+Math.round(colorScale(d))+')')
          .classed('view',true);
      }
    })
  legend.append('g')
    .attr('transform','translate(0,'+legendY+')')
    .call(legendAxis)
    .select('.domain').remove(); //removes the base line of the axis
  
  //draw map
  svg.append('g')
    .selectAll('path')
    //the dataset is already in topojson format, use topojson library
    .data(topojson.feature(data.geo,data.geo.objects.counties).features)
    .enter()
    .append('path')
    .attr('class',d=>'county view county-'+Math.round(colorScale(data.edu.filter(o=>o.fips==d.id)[0].bachelorsOrHigher)))
    .attr('data-fips',d=>d.id)
    .attr('data-education',d=>data.edu.filter(o=>o.fips==d.id)[0].bachelorsOrHigher)
    .style('fill',d=>colorHSL(themeHue,colorScale(data.edu.filter(o=>o.fips==d.id)[0].bachelorsOrHigher)))
    .attr('d',d3.geoPath())
    .on('mouseenter',function(d){
      //only allow tooltip and animation if area has .view class
      if (d3.select(this).attr('class').includes('view')){
        d3.select(this).style('stroke','black');
        //legend area effect on county hover
        d3.select('#interval-'+Math.round(colorScale(data.edu.filter(o=>o.fips==d.id)[0].bachelorsOrHigher)))
          .attr('y',legendY-10)
          .attr('height',legendH+10);
        //tooltip
        let tip=d3.select('#svg-container').append('div')
          .attr('id','tooltip')
          .attr('class','tooltip')
          .style('left',(d3.event.pageX-60)+'px')
          .style('top',(d3.event.pageY-60)+'px')
        tip.append('p')
          .html(data.edu.filter(o=>o.fips==d.id)[0].area_name + ', ' + data.edu.filter(o=>o.fips==d.id)[0].state);
        tip.append('p')
          .html(data.edu.filter(o=>o.fips==d.id)[0].bachelorsOrHigher + '%');
      }
    })
    .on('mouseleave',function(d){
      //only allow tooltip and animation if area has .view class
      if(d3.select(this).attr('class').includes('view')){
        d3.select(this).style('stroke','none');
        d3.selectAll('.tooltip').remove();
        //only animate legend if not viewing a category
        if(!viewCategory){
          d3.select('#interval-'+Math.round(colorScale(data.edu.filter(o=>o.fips==d.id)[0].bachelorsOrHigher)))
            .attr('y',legendY)
            .attr('height',legendH);
        }
      }
    })
  //draw states
  svg.append('path')
    .datum(topojson.mesh(data.geo,data.geo.objects.states),(a,b)=>a!==b)
    .attr('class','states')
    .attr('d',d3.geoPath());
  
}
              
            
!
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