Basic hexbin chart




This post describes how to build a very basic hexbin chart with d3.js. The hexbin chart is a representation of a 2d density. 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-hexbin plugin allows to group two-dimensional points into hexagonal bins using the d3.hexbin(). You can control the hexbin size with the radius() argument.

  • Finally, paths can be drawn using .hexagon()
<!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-hexbin.v0.2.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, 18])
    .range([ 0, width ]);
  svg.append("g")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(x));

  // Add Y axis
  var y = d3.scaleLinear()
    .domain([5, 20])
    .range([ height, 0 ]);
  svg.append("g")
    .call(d3.axisLeft(y));

  // Reformat the data: d3.hexbin() needs a specific format
  var inputForHexbinFun = []
  data.forEach(function(d) {
    inputForHexbinFun.push( [x(d.x), y(d.y)] )  // Note that we had the transform value of X and Y !
  })

  // Prepare a color palette
  var color = d3.scaleLinear()
      .domain([0, 600]) // Number of points in the bin?
      .range(["transparent",  "#69b3a2"])

  // Compute the hexbin data
  var hexbin = d3.hexbin()
    .radius(9) // size of the bin in px
    .extent([ [0, 0], [width, height] ])

  // Plot the hexbins
  svg.append("clipPath")
      .attr("id", "clip")
    .append("rect")
      .attr("width", width)
      .attr("height", height)

  svg.append("g")
    .attr("clip-path", "url(#clip)")
    .selectAll("path")
    .data( hexbin(inputForHexbinFun) )
    .enter().append("path")
      .attr("d", hexbin.hexagon())
      .attr("transform", function(d) { return "translate(" + d.x + "," + d.y + ")"; })
      .attr("fill", function(d) { return color(d.length); })
      .attr("stroke", "black")
      .attr("stroke-width", "0.1")
})
</script>

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