Recreation of Dupin's Choropleth from 1826

<!DOCTYPE html>
<meta charset="utf-8">
<style>

.border{
  stroke-width:.3px;
  fill:none;
  stroke:#333;
}
svg {
  font: 10px sans-serif;
}
.caption{
  font-weight: bold;
  fill: black;
}
.key path {
  display: none;
}
.key line{
  stroke:#000;
  shape-rendering:crispEdges;
}

</style>
<body>
<svg width="960" height="500"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://d3js.org/topojson.v1.min.js"></script>
<script>

var width = 960;
var height = 500;

var colorArray = ['#000000', '#3f3f3f', '#7e7e7e', '#d3d3d3', 'white'];

var svg = d3.select("svg")
    .attr("width", width)
    .attr("height", height);

var color = d3.scaleThreshold()
  .domain([5, 8, 9, 20, 38])
  .range(colorArray);

// A position encoding for the key only.
var x = d3.scaleLinear()
  .domain([0, 38])
  .range([0, 200]);

var xAxis = d3.axisBottom()
  .scale(x)
  .tickSize(13)
  .tickValues(color.domain());

var projection = d3.geoAlbers()
  .center([2, 49.5])
  .rotate([-2.8, 3])
  .parallels([45, 55])
  .scale(2500)
  .translate([width / 2, height / 2]);

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

d3.json("france.json", function(error1, france) {
	d3.csv("data.csv", function(error2, illiteracy) {
    if (error1) throw error1;
    if (error2) throw error2;

var g = svg.append("g")
        .attr("class", "key")
        .attr("transform", "translate(40,40)");

g.selectAll("rect")
      .data(color.range().map((currentColor) => {
        var d = color.invertExtent(currentColor);
        if (d[0] == null) d[0] = x.domain()[0];
        if (d[1] == null) d[1] = x.domain()[1];
        return d;
      }))
      .enter()
      .append('rect')
      .attr('height', 8)
      .attr('x', d => x(d[0]))
      .attr('width', d => x(d[1]) - x(d[0]))
      .style('fill', d => color(d[0]));

g.call(xAxis).append('text')
      .attr('class', 'caption')
      .attr('dy',-10)
    	.attr('dx', 30)
      .text('Illiteracy (%)');

svg.selectAll('.departements')
      .data(topojson.feature(france, france.objects.regions).features)
      .enter().append('path')
      .attr('class', 'departements')
      .attr('d', path)
      .style('fill', (departement) => {
        var paringData = illiteracy.filter((illiteracy) => {
          return departement.properties.name === illiteracy.name;
        })[0];
        return paringData ? color(paringData.illiteracy) : color(0);
      });

svg.append('path')
      .datum(topojson.mesh(france, france.objects.regions, (a, b) => { return a.properties.name !== b.properties.name || a === b; }))
      .attr('class','border')
      .attr('d', path);

});  
});

</script>

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

https://d3js.org/topojson.v1.min.js