Density 2d with shading in d3.js




This post describes how to build a very basic 2d density with shading with d3.js. The hexbin chart is a representation of a 2d density, very close from a contour plot but with shading between layers. It is made thanks to the d3-contour plugin. Visit data-to-viz.com for more theoretical info on 2d density charts.


2d density section

Steps:

  • The Html part of the code just creates a div that will be modified by d3 later on. It also load the d3-hexbin plugin

  • The input dataset has 2 numeric variables: x and y. A 2d density plot basically represents the amount of data point on each part of the grid.

  • The d3-contour plugin allows to compute the 2d density using the contourDensity() function.

  • Finally, paths can be drawn using .geoPath()
<!DOCTYPE html>
<meta charset="utf-8">

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

<!-- specific plugin -->
<script src="https://d3js.org/d3-contour.v1.min.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: 10, right: 30, bottom: 30, left: 40},
    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 + ")");

// read data
d3.csv("https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/data_for_density2d.csv", function(data) {

  // Add X axis
  var x = d3.scaleLinear()
    .domain([5, 20])
    .range([ margin.left, width - margin.right ]);
  svg.append("g")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(x));

  // Add Y axis
  var y = d3.scaleLinear()
    .domain([5, 25])
    .range([ height - margin.bottom, margin.top ]);
  svg.append("g")
    .call(d3.axisLeft(y));

  // Prepare a color palette
  var color = d3.scaleLinear()
      .domain([0, 1]) // Points per square pixel.
      .range(["white", "#69b3a2"])

  // compute the density data
  var densityData = d3.contourDensity()
    .x(function(d) { return x(d.x); })
    .y(function(d) { return y(d.y); })
    .size([width, height])
    .bandwidth(20)
    (data)

  // show the shape!
  svg.insert("g", "g")
    .selectAll("path")
    .data(densityData)
    .enter().append("path")
      .attr("d", d3.geoPath())
      .attr("fill", function(d) { return color(d.value); })
})
</script>

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