Density plot with several groups in d3.js





This post describes how to build a density chart with several groups using d3.js. It follows the previous basic density chart and is useful to compare the distribution of several variables. More density chart examples in the density chart section of the gallery.


Density plot section

Notes:

  • This chart is an extension of the previous basic density chart.

  • It splits the dataset in 2 parts thanks to the filter() function. See more on data manipulation. Density is estimated and plotted for each part.

  • This posts also shows how to add a basic legend to the chart

  • Todo: code is currently hardcoded for 2 groups. It would be better to loop on every group of the dataset.
<!DOCTYPE html>
<meta charset="utf-8">

<!-- Load d3.js -->
<script src="http://d3js.org/d3.v4.js"></script>

<!-- Create a div where the graph will take place -->
<div id="my_dataviz"></div>

<script>

// set the dimensions and margins of the graph
var margin = {top: 30, right: 30, bottom: 30, left: 50},
    width = 460 - margin.left - margin.right,
    height = 400 - margin.top - margin.bottom;

// append the svg object to the body of the page
var svg = d3.select("#my_dataviz")
  .append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform",
          "translate(" + margin.left + "," + margin.top + ")");

// get the data
d3.csv("https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/data_doubleHist.csv", function(data) {

  // add the x Axis
  var x = d3.scaleLinear()
      .domain([-10,15])
      .range([0, width]);
  svg.append("g")
      .attr("transform", "translate(0," + height + ")")
      .call(d3.axisBottom(x));

  // add the y Axis
  var y = d3.scaleLinear()
            .range([height, 0])
            .domain([0, 0.12]);
  svg.append("g")
      .call(d3.axisLeft(y));

  // Compute kernel density estimation
  var kde = kernelDensityEstimator(kernelEpanechnikov(7), x.ticks(60))
  var density1 =  kde( data
      .filter( function(d){return d.type === "variable 1"} )
      .map(function(d){  return d.value; }) )
  var density2 =  kde( data
      .filter( function(d){return d.type === "variable 2"} )
      .map(function(d){  return d.value; }) )

  // Plot the area
  svg.append("path")
      .attr("class", "mypath")
      .datum(density1)
      .attr("fill", "#69b3a2")
      .attr("opacity", ".6")
      .attr("stroke", "#000")
      .attr("stroke-width", 1)
      .attr("stroke-linejoin", "round")
      .attr("d",  d3.line()
        .curve(d3.curveBasis)
          .x(function(d) { return x(d[0]); })
          .y(function(d) { return y(d[1]); })
      );

  // Plot the area
  svg.append("path")
      .attr("class", "mypath")
      .datum(density2)
      .attr("fill", "#404080")
      .attr("opacity", ".6")
      .attr("stroke", "#000")
      .attr("stroke-width", 1)
      .attr("stroke-linejoin", "round")
      .attr("d",  d3.line()
        .curve(d3.curveBasis)
          .x(function(d) { return x(d[0]); })
          .y(function(d) { return y(d[1]); })
      );

});

// Handmade legend
svg.append("circle").attr("cx",300).attr("cy",30).attr("r", 6).style("fill", "#69b3a2")
svg.append("circle").attr("cx",300).attr("cy",60).attr("r", 6).style("fill", "#404080")
svg.append("text").attr("x", 320).attr("y", 30).text("variable A").style("font-size", "15px").attr("alignment-baseline","middle")
svg.append("text").attr("x", 320).attr("y", 60).text("variable B").style("font-size", "15px").attr("alignment-baseline","middle")

// Function to compute density
function kernelDensityEstimator(kernel, X) {
  return function(V) {
    return X.map(function(x) {
      return [x, d3.mean(V, function(v) { return kernel(x - v); })];
    });
  };
}
function kernelEpanechnikov(k) {
  return function(v) {
    return Math.abs(v /= k) <= 1 ? 0.75 * (1 - v * v) / k : 0;
  };
}

</script>

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