Pie chart with annotation




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


Pie chart section

Steps:

  • Warning: are you sure you want to use a pie chart? Learn why it is not recommended.

  • Start by learning how to build the most basic pie chart.

  • Here the label positions are computed thanks to the centroid() function.

  • Note that a color scale is used for group, as explained here.
<!DOCTYPE html>
<meta charset="utf-8">

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

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

<!-- Color scale -->
<script src="https://d3js.org/d3-scale-chromatic.v1.min.js"></script>


<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 subtract 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}

// set the color scale
var color = d3.scaleOrdinal()
  .domain(data)
  .range(d3.schemeSet2);

// Compute the position of each group on the pie:
var pie = d3.pie()
  .value(function(d) {return d.value; })
var data_ready = pie(d3.entries(data))
// Now I know that group A goes from 0 degrees to x degrees and so on.

// shape helper to build arcs:
var arcGenerator = d3.arc()
  .innerRadius(0)
  .outerRadius(radius)

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

// Now add the annotation. Use the centroid method to get the best coordinates
svg
  .selectAll('mySlices')
  .data(data_ready)
  .enter()
  .append('text')
  .text(function(d){ return "grp " + d.data.key})
  .attr("transform", function(d) { return "translate(" + arcGenerator.centroid(d) + ")";  })
  .style("text-anchor", "middle")
  .style("font-size", 17)


</script>

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