Add tooltip to bubble chart in d3.js

Steps:

First of all, you need to understand how to build a basic bubble chart.

This post aims to show how to add tooltip. The technique is always almost the same:

Create a tooltip element that will contain the text. Hide it with css (use opacity or display)
Create 3 functions that show the tooltip (showTooltip), update its position (moveTooltip) and hide it when mouse leave (hideTooltip)
Trigger these functions when the user hover / unhover the circles.

Note: It is a good practice to change circle feature on hover, to make sure what data point you are looking at. Here it is done with css, changing the stroke color.

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

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

<!-- Load color scale -->
<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>

<!-- A bit of CSS: change stroke color of circle on hover (white -> black) -->
<style>
.bubbles {
  stroke-width: 2px;
  stroke: white;
}
.bubbles:hover {
  stroke: black;
}
</style>

<script>

// set the dimensions and margins of the graph
var margin = {top: 10, right: 20, bottom: 30, left: 50},
    width = 500 - margin.left - margin.right,
    height = 420 - 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/4_ThreeNum.csv", function(data) {

  // Add X axis
  var x = d3.scaleLinear()
    .domain([0, 12000])
    .range([ 0, width ]);
  svg.append("g")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(x));

  // Add Y axis
  var y = d3.scaleLinear()
    .domain([35, 90])
    .range([ height, 0]);
  svg.append("g")
    .call(d3.axisLeft(y));

  // Add a scale for bubble size
  var z = d3.scaleLinear()
    .domain([200000, 1310000000])
    .range([ 4, 40]);

  // Add a scale for bubble color
  var myColor = d3.scaleOrdinal()
    .domain(["Asia", "Europe", "Americas", "Africa", "Oceania"])
    .range(d3.schemeSet2);

  // -1- Create a tooltip div that is hidden by default:
  var tooltip = d3.select("#my_dataviz")
    .append("div")
      .style("opacity", 0)
      .attr("class", "tooltip")
      .style("background-color", "black")
      .style("border-radius", "5px")
      .style("padding", "10px")
      .style("color", "white")

  // -2- Create 3 functions to show / update (when mouse move but stay on same circle) / hide the tooltip
  var showTooltip = function(d) {
    tooltip
      .transition()
      .duration(200)
    tooltip
      .style("opacity", 1)
      .html("Country: " + d.country)
      .style("left", (d3.mouse(this)[0]+30) + "px")
      .style("top", (d3.mouse(this)[1]+30) + "px")
  }
  var moveTooltip = function(d) {
    tooltip
      .style("left", (d3.mouse(this)[0]+30) + "px")
      .style("top", (d3.mouse(this)[1]+30) + "px")
  }
  var hideTooltip = function(d) {
    tooltip
      .transition()
      .duration(200)
      .style("opacity", 0)
  }

  // Add dots
  svg.append('g')
    .selectAll("dot")
    .data(data)
    .enter()
    .append("circle")
      .attr("class", "bubbles")
      .attr("cx", function (d) { return x(d.gdpPercap); } )
      .attr("cy", function (d) { return y(d.lifeExp); } )
      .attr("r", function (d) { return z(d.pop); } )
      .style("fill", function (d) { return myColor(d.continent); } )
    // -3- Trigger the functions
    .on("mouseover", showTooltip )
    .on("mousemove", moveTooltip )
    .on("mouseleave", hideTooltip )

  })
</script>

Steps:

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