Donut chart with group label in d3.js





This post describes how to build a donut chart with group labels in d3.js. You can see many other examples in the donut chart section of the gallery. Watch out: pie and donut chart are most of the time a very bad way to convey information as explained in this post.


Donut chart section

Steps:

  • Disclaimer: strongly inspired by this block by Laxmikanta Nayak.

  • Start by understanding how to build a basic donut chart.

  • Here the tricky part is to find the 3 x,y coordinates we need to add the labels:
    • posA: line insertion in the slice
    • posB: line break position
    • posC: label position

  • posA is straightforward thanks to the centroid function of d3. It is used to label pie chart as well.

  • To find the position of posB, another arc generator called outerArc is created. It is not used to draw arcs, but only to compute centroid.

  • posC has the same Y coordinates than posB. However its x coordinate is at the extreme part of the svg area.
<!DOCTYPE html>
<meta charset="utf-8">

<!-- Load d3.js & color scale -->
<script src="https://d3js.org/d3.v4.js"></script>
<script src="https://d3js.org/d3-scale-chromatic.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 width = 450
    height = 450
    margin = 40

// The radius of the pieplot is half the width or half the height (smallest one). I substract a bit of margin.
var radius = Math.min(width, height) / 2 - margin

// append the svg object to the div called 'my_dataviz'
var svg = d3.select("#my_dataviz")
  .append("svg")
    .attr("width", width)
    .attr("height", height)
  .append("g")
    .attr("transform", "translate(" + width / 2 + "," + height / 2 + ")");

// Create dummy data
var data = {a: 9, b: 20, c:30, d:8, e:12, f:3, g:7, h:14}

// set the color scale
var color = d3.scaleOrdinal()
  .domain(["a", "b", "c", "d", "e", "f", "g", "h"])
  .range(d3.schemeDark2);

// Compute the position of each group on the pie:
var pie = d3.pie()
  .sort(null) // Do not sort group by size
  .value(function(d) {return d.value; })
var data_ready = pie(d3.entries(data))

// The arc generator
var arc = d3.arc()
  .innerRadius(radius * 0.5)         // This is the size of the donut hole
  .outerRadius(radius * 0.8)

// Another arc that won't be drawn. Just for labels positionning
var outerArc = d3.arc()
  .innerRadius(radius * 0.9)
  .outerRadius(radius * 0.9)

// Build the pie chart: Basically, each part of the pie is a path that we build using the arc function.
svg
  .selectAll('allSlices')
  .data(data_ready)
  .enter()
  .append('path')
  .attr('d', arc)
  .attr('fill', function(d){ return(color(d.data.key)) })
  .attr("stroke", "white")
  .style("stroke-width", "2px")
  .style("opacity", 0.7)

// Add the polylines between chart and labels:
svg
  .selectAll('allPolylines')
  .data(data_ready)
  .enter()
  .append('polyline')
    .attr("stroke", "black")
    .style("fill", "none")
    .attr("stroke-width", 1)
    .attr('points', function(d) {
      var posA = arc.centroid(d) // line insertion in the slice
      var posB = outerArc.centroid(d) // line break: we use the other arc generator that has been built only for that
      var posC = outerArc.centroid(d); // Label position = almost the same as posB
      var midangle = d.startAngle + (d.endAngle - d.startAngle) / 2 // we need the angle to see if the X position will be at the extreme right or extreme left
      posC[0] = radius * 0.95 * (midangle < Math.PI ? 1 : -1); // multiply by 1 or -1 to put it on the right or on the left
      return [posA, posB, posC]
    })

// Add the polylines between chart and labels:
svg
  .selectAll('allLabels')
  .data(data_ready)
  .enter()
  .append('text')
    .text( function(d) { console.log(d.data.key) ; return d.data.key } )
    .attr('transform', function(d) {
        var pos = outerArc.centroid(d);
        var midangle = d.startAngle + (d.endAngle - d.startAngle) / 2
        pos[0] = radius * 0.99 * (midangle < Math.PI ? 1 : -1);
        return 'translate(' + pos + ')';
    })
    .style('text-anchor', function(d) {
        var midangle = d.startAngle + (d.endAngle - d.startAngle) / 2
        return (midangle < Math.PI ? 'start' : 'end')
    })

</script>

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