Line plot with several groups





This post describes how to build a line chart with several groups. Mind the Spaghetti chart: it becomes unreadable when too many lines are displayed. More examples of line charts in the dedicated section. This example works with d3.js v4 and v6


Line chart section

Steps:

  • First of all, it is important to understand how to build a basic line chart with d3.js.

  • Here, the data is in long (or tidy) format: one numerical columns provides the information of each group.

  • Thus, the first step is to use the d3.nest function to group the variable. Read more about it here.

  • Next steps are pretty usual: building axis, color scales and lines.
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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: 10, right: 30, bottom: 30, left: 60},
    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 the data
d3.csv("https://raw.githubusercontent.com/holtzy/data_to_viz/master/Example_dataset/5_OneCatSevNumOrdered.csv", function(data) {

  // group the data: I want to draw one line per group
  var sumstat = d3.nest() // nest function allows to group the calculation per level of a factor
    .key(function(d) { return d.name;})
    .entries(data);

  // Add X axis --> it is a date format
  var x = d3.scaleLinear()
    .domain(d3.extent(data, function(d) { return d.year; }))
    .range([ 0, width ]);
  svg.append("g")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(x).ticks(5));

  // Add Y axis
  var y = d3.scaleLinear()
    .domain([0, d3.max(data, function(d) { return +d.n; })])
    .range([ height, 0 ]);
  svg.append("g")
    .call(d3.axisLeft(y));

  // color palette
  var res = sumstat.map(function(d){ return d.key }) // list of group names
  var color = d3.scaleOrdinal()
    .domain(res)
    .range(['#e41a1c','#377eb8','#4daf4a','#984ea3','#ff7f00','#ffff33','#a65628','#f781bf','#999999'])

  // Draw the line
  svg.selectAll(".line")
      .data(sumstat)
      .enter()
      .append("path")
        .attr("fill", "none")
        .attr("stroke", function(d){ return color(d.key) })
        .attr("stroke-width", 1.5)
        .attr("d", function(d){
          return d3.line()
            .x(function(d) { return x(d.year); })
            .y(function(d) { return y(+d.n); })
            (d.values)
        })

})

</script>
<script>

// set the dimensions and margins of the graph
const margin = {top: 10, right: 30, bottom: 30, left: 60},
    width = 460 - margin.left - margin.right,
    height = 400 - 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 the data
d3.csv("https://raw.githubusercontent.com/holtzy/data_to_viz/master/Example_dataset/5_OneCatSevNumOrdered.csv").then( function(data) {

  // group the data: I want to draw one line per group
  const sumstat = d3.group(data, d => d.name); // nest function allows to group the calculation per level of a factor

  // Add X axis --> it is a date format
  const x = d3.scaleLinear()
    .domain(d3.extent(data, function(d) { return d.year; }))
    .range([ 0, width ]);
  svg.append("g")
    .attr("transform", `translate(0, ${height})`)
    .call(d3.axisBottom(x).ticks(5));

  // Add Y axis
  const y = d3.scaleLinear()
    .domain([0, d3.max(data, function(d) { return +d.n; })])
    .range([ height, 0 ]);
  svg.append("g")
    .call(d3.axisLeft(y));

  // color palette
  const color = d3.scaleOrdinal()
    .range(['#e41a1c','#377eb8','#4daf4a','#984ea3','#ff7f00','#ffff33','#a65628','#f781bf','#999999'])

  // Draw the line
  svg.selectAll(".line")
      .data(sumstat)
      .join("path")
        .attr("fill", "none")
        .attr("stroke", function(d){ return color(d[0]) })
        .attr("stroke-width", 1.5)
        .attr("d", function(d){
          return d3.line()
            .x(function(d) { return x(d.year); })
            .y(function(d) { return y(+d.n); })
            (d[1])
        })

})
</script>

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