Most basic arc diagram in d3.js




This post describes how to build a very basic arc diagram with d3.js. It represent a very basic network composed of 6 nodes. You can see many other examples in the arc diagram section of the gallery. Learn more about the theory of arc diagrams in data-to-viz.com.


Arc diagram section

Steps:

  • Data input format is Json. This document explains how to get this format from a classic table.

  • Plotting the nodes is pretty straightforward: a scalePoint() is set to distribute them uniformly along the X axis. They are then added using a classing append("circle") approach.

  • Drawing links is a bit tricky. Code is highly commented and should allow to understand the basic process.
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<!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>

<!DOCTYPE html>
<meta charset="utf-8">

<!-- Load d3.js -->
<script src="https://d3js.org/d3.v6.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: 20, right: 30, bottom: 20, left: 30},
  width = 450 - margin.left - margin.right,
  height = 300 - 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 dummy data
d3.json("https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/data_network.json", function( data) {

  // List of node names
  var allNodes = data.nodes.map(function(d){return d.name})

  // A linear scale to position the nodes on the X axis
  var x = d3.scalePoint()
    .range([0, width])
    .domain(allNodes)

  // Add the circle for the nodes
  svg
    .selectAll("mynodes")
    .data(data.nodes)
    .enter()
    .append("circle")
      .attr("cx", function(d){ return(x(d.name))})
      .attr("cy", height-30)
      .attr("r", 8)
      .style("fill", "#69b3a2")

  // And give them a label
  svg
    .selectAll("mylabels")
    .data(data.nodes)
    .enter()
    .append("text")
      .attr("x", function(d){ return(x(d.name))})
      .attr("y", height-10)
      .text(function(d){ return(d.name)})
      .style("text-anchor", "middle")

  // Add links between nodes. Here is the tricky part.
  // In my input data, links are provided between nodes -id-, NOT between node names.
  // So I have to do a link between this id and the name
  var idToNode = {};
  data.nodes.forEach(function (n) {
    idToNode[n.id] = n;
  });
  // Cool, now if I do idToNode["2"].name I've got the name of the node with id 2

  // Add the links
  svg
    .selectAll('mylinks')
    .data(data.links)
    .enter()
    .append('path')
    .attr('d', function (d) {
      start = x(idToNode[d.source].name)    // X position of start node on the X axis
      end = x(idToNode[d.target].name)      // X position of end node
      return ['M', start, height-30,    // the arc starts at the coordinate x=start, y=height-30 (where the starting node is)
        'A',                            // This means we're gonna build an elliptical arc
        (start - end)/2, ',',    // Next 2 lines are the coordinates of the inflexion point. Height of this point is proportional with start - end distance
        (start - end)/2, 0, 0, ',',
        start < end ? 1 : 0, end, ',', height-30] // We always want the arc on top. So if end is before start, putting 0 here turn the arc upside down.
        .join(' ');
    })
    .style("fill", "none")
    .attr("stroke", "black")

})

</script>
<script>

// set the dimensions and margins of the graph
const margin = {top: 20, right: 30, bottom: 20, left: 30},
  width = 450 - margin.left - margin.right,
  height = 300 - margin.top - margin.bottom;

// append the svg object to the body of the page
const 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 dummy data
d3.json("https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/data_network.json").then(function(data) {

  // List of node names
  var allNodes = data.nodes.map(d => d.name)

  // A linear scale to position the nodes on the X axis
  var x = d3.scalePoint()
    .range([0, width])
    .domain(allNodes)

  // Add the circle for the nodes
  svg
    .selectAll("mynodes")
    .data(data.nodes)
    .enter()
    .append("circle")
      .attr("cx", d => x(d.name))
      .attr("cy", height-30)
      .attr("r", 8)
      .style("fill", "#69b3a2")

  // And give them a label
  svg
    .selectAll("mylabels")
    .data(data.nodes)
    .enter()
    .append("text")
      .attr("x", d => x(d.name))
      .attr("y", height-10)
      .text(d => d.name)
      .style("text-anchor", "middle")

  // Add links between nodes. Here is the tricky part.
  // In my input data, links are provided between nodes -id-, NOT between node names.
  // So I have to do a link between this id and the name
  var idToNode = {};
  data.nodes.forEach(node =>
    idToNode[node.id] = node
  );
  // Cool, now if I do idToNode["2"].name I've got the name of the node with id 2

  // Add the links
  svg
    .selectAll('mylinks')
    .data(data.links)
    .enter()
    .append('path')
    .attr('d', function (d) {
      start = x(idToNode[d.source].name)    // X position of start node on the X axis
      end = x(idToNode[d.target].name)      // X position of end node
      return ['M', start, height-30,    // the arc starts at the coordinate x=start, y=height-30 (where the starting node is)
        'A',                            // This means we're gonna build an elliptical arc
        (start - end)/2, ',',    // Next 2 lines are the coordinates of the inflexion point. Height of this point is proportional with start - end distance
        (start - end)/2, 0, 0, ',',
        start < end ? 1 : 0, end, ',', height-30] // We always want the arc on top. So if end is before start, putting 0 here turn the arc upside down.
        .join(' ');
    })
    .style("fill", "none")
    .attr("stroke", "black")
})

</script>

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