US Choropleth

A modular version of this excellent choropleth - will map any CSV statesdata.csv on a linear continuous value, using d3js v4.

Requirements for statesdata.csv:

state column with standard full-length state names
value column with any numeric values

Also, you can edit the color ramp using the lowColor and highColor variables.

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<script src="https://d3js.org/d3.v4.min.js"></script>
<style type="text/css">

/* Legend Font Style */
body {
	font: 11px sans-serif;
	background-color: #ffffff;
}
        
/* Legend Position Style */
.legend {
	position:absolute;
	left:20px;
	top:50px;
}

.axis text {
	font: 10px sans-serif;
}

.axis line, .axis path {
	fill: none;
	stroke: #000;
	shape-rendering: crispEdges;
}

</style>
</head>
<body>
<script type="text/javascript">
		
//Width and height of map
var width = 1200;
var height = 700;

var lowColor = '#B2FFB2'
var highColor = '#1B4D3E'

// D3 Projection
var projection = d3.geoAlbersUsa()
  .translate([width / 1.75, height / 2]) // translate to center of screen
  .scale([1200]); // scale things down so see entire US

// Define path generator
var path = d3.geoPath() // path generator that will convert GeoJSON to SVG paths
  .projection(projection); // tell path generator to use albersUsa projection

//Create SVG element and append map to the SVG
var svg = d3.select("body")
  .append("svg")
  .attr("width", width)
  .attr("height", height);

// Add title
var title = svg.append('text')
	.text('Wealth Inequality in the US by State')
	.attr('y', 40)
	.attr('x', width / 3.25)
	.attr('font-size', '40px')
	.attr('opacity', 0.75)

// Load in my states data!
d3.csv("statesdata.csv", function(data) {
	var dataArray = [];
	for (var d = 0; d < data.length; d++) {
		dataArray.push(parseFloat(data[d].value))
	}
	var minVal = d3.min(dataArray)
	var maxVal = d3.max(dataArray)
	var ramp = d3.scaleLinear().domain([minVal,maxVal]).range([lowColor,highColor])
	
  // Load GeoJSON data and merge with states data
  d3.json("us-states.json", function(json) {

// Loop through each state data value in the .csv file
    for (var i = 0; i < data.length; i++) {

// Grab State Name
      var dataState = data[i].state;

// Grab data value 
      var dataValue = data[i].value;

// Find the corresponding state inside the GeoJSON
      for (var j = 0; j < json.features.length; j++) {
        var jsonState = json.features[j].properties.name;

if (dataState == jsonState) {

// Copy the data value into the JSON
          json.features[j].properties.value = dataValue;

// Stop looking through the JSON
          break;
        }
      }
    }

// Bind the data to the SVG and create one path per GeoJSON feature
    svg.selectAll("path")
      .data(json.features)
      .enter()
      .append("path")
      .attr("d", path)
      .style("stroke", "#fff")
      .style("stroke-width", "1")
      .style("fill", function(d) { return ramp(d.properties.value) });
    
		// add a legend
		var w = 140, h = 600;

var key = d3.select("body")
			.append("svg")
			.attr("width", w)
			.attr("height", h)
			.attr("class", "legend");
		
		var legend = key.append("defs")
			.append("svg:linearGradient")
			.attr("id", "gradient")
			.attr("x1", "100%")
			.attr("y1", "0%")
			.attr("x2", "100%")
			.attr("y2", "100%")
			.attr("spreadMethod", "pad");

legend.append("stop")
			.attr("offset", "0%")
			.attr("stop-color", highColor)
			.attr("stop-opacity", 1);
			
		legend.append("stop")
			.attr("offset", "100%")
			.attr("stop-color", lowColor)
			.attr("stop-opacity", 1);

key.append("rect")
			.attr("width", w - 100)
			.attr("height", h)
			.style("fill", "url(#gradient)")
			.attr("transform", "translate(0,10)");

var y = d3.scaleLinear()
			.range([h, 0])
			.domain([minVal, maxVal]);

var yAxis = d3.axisRight(y);

key.append("g")
			.attr("class", "y axis")
			.attr("transform", "translate(41,10)")
			.call(yAxis)

// add legend info
		svg.append('text')
			.text('Most Unequal')
			.attr('font-size', '12px')
		  	.attr('y', 40)
			.attr('x', 15)

svg.append('text')
			.text('Gini Coefficient')
			.attr('font-size', '10px')
			.attr('y', 65)
			.attr('x', 85)
  });
});
</script>
</body>
</html>

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