Heatmap with tooltip in d3.js

This post follows the previous one on basic heatmaps. It shows how to add a tooltip to every cell of the chart to display exact values and any additional text.

Heatmap section


  • The Html part of the code just creates a div that will be modified by d3 later on.

  • The first part of the javascript code set a svg area. It specify the chart size and its margin. Read more.

  • A dummy dataset has been created for this example, at the long format. (3 columns: row, col, value).

  • The first step is to build scales and axis. Thus each entity will have a position on the grid.

  • A scaleBand() is used, which allows to control the size of each square using the padding property. If padding is close from 1, square are very small. (0 for very big)

  • Finally, each square can be added using a rect element.
<!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>


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

// append the svg object to the body of the page
var svg = d3.select("#my_dataviz")
  .attr("width", width + margin.left + margin.right)
  .attr("height", height + margin.top + margin.bottom)
        "translate(" + margin.left + "," + margin.top + ")");

// Labels of row and columns
var myGroups = ["A", "B", "C", "D", "E", "F", "G", "H", "I", "J"]
var myVars = ["v1", "v2", "v3", "v4", "v5", "v6", "v7", "v8", "v9", "v10"]

// Build X scales and axis:
var x = d3.scaleBand()
  .range([ 0, width ])
  .attr("transform", "translate(0," + height + ")")

// Build X scales and axis:
var y = d3.scaleBand()
  .range([ height, 0 ])

// Build color scale
var myColor = d3.scaleLinear()
  .range(["white", "#69b3a2"])

//Read the data
d3.csv("https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/heatmap_data.csv", function(data) {

  // create a tooltip
  var tooltip = d3.select("#my_dataviz")
    .style("opacity", 0)
    .attr("class", "tooltip")
    .style("background-color", "white")
    .style("border", "solid")
    .style("border-width", "2px")
    .style("border-radius", "5px")
    .style("padding", "5px")

  // Three function that change the tooltip when user hover / move / leave a cell
  var mouseover = function(d) {
    tooltip.style("opacity", 1)
  var mousemove = function(d) {
      .html("The exact value of<br>this cell is: " + d.value)
      .style("left", (d3.mouse(this)[0]+70) + "px")
      .style("top", (d3.mouse(this)[1]) + "px")
  var mouseleave = function(d) {
    tooltip.style("opacity", 0)

  // add the squares
    .data(data, function(d) {return d.group+':'+d.variable;})
      .attr("x", function(d) { return x(d.group) })
      .attr("y", function(d) { return y(d.variable) })
      .attr("width", x.bandwidth() )
      .attr("height", y.bandwidth() )
      .style("fill", function(d) { return myColor(d.value)} )
    .on("mouseover", mouseover)
    .on("mousemove", mousemove)
    .on("mouseleave", mouseleave)


Related blocks →