Histogram with variable bin size




This post describes how to build a histogram with d3.js and add a slider that controls bin size. It is important to play with bin size since the conclusion can become significantly different.


Histogram section

Steps:

  • 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 histogram displays the distribution of a numeric variable. So you basically need only one column in your input file. (see input here)

  • This numeric variable is provided to the d3.histogram() function that will compute the binning and returns the coordinates of each bar.

  • These bars can thus be drawn using a classic .append("rect") approach.
<!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>

<p>
  <label># bins</label>
  <input type="number" min="1" max="100" step="30" value="20" id="nBin">
</p>


<script>

// set the dimensions and margins of the graph
var margin = {top: 10, right: 30, bottom: 30, left: 40},
    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 + ")");

// get the data
d3.csv("https://raw.githubusercontent.com/holtzy/data_to_viz/master/Example_dataset/1_OneNum.csv", function(data) {

  // X axis: scale and draw:
  var x = d3.scaleLinear()
      .domain([0, 1000])     // can use this instead of 1000 to have the max of data: d3.max(data, function(d) { return +d.price })
      .range([0, width]);
  svg.append("g")
      .attr("transform", "translate(0," + height + ")")
      .call(d3.axisBottom(x));

  // Y axis: initialization
  var y = d3.scaleLinear()
      .range([height, 0]);
  var yAxis = svg.append("g")

  // A function that builds the graph for a specific value of bin
  function update(nBin) {

    // set the parameters for the histogram
    var histogram = d3.histogram()
        .value(function(d) { return d.price; })   // I need to give the vector of value
        .domain(x.domain())  // then the domain of the graphic
        .thresholds(x.ticks(nBin)); // then the numbers of bins

    // And apply this function to data to get the bins
    var bins = histogram(data);

    // Y axis: update now that we know the domain
    y.domain([0, d3.max(bins, function(d) { return d.length; })]);   // d3.hist has to be called before the Y axis obviously
    yAxis
        .transition()
        .duration(1000)
        .call(d3.axisLeft(y));

    // Join the rect with the bins data
    var u = svg.selectAll("rect")
        .data(bins)

    // Manage the existing bars and eventually the new ones:
    u
        .enter()
        .append("rect") // Add a new rect for each new elements
        .merge(u) // get the already existing elements as well
        .transition() // and apply changes to all of them
        .duration(1000)
          .attr("x", 1)
          .attr("transform", function(d) { return "translate(" + x(d.x0) + "," + y(d.length) + ")"; })
          .attr("width", function(d) { return x(d.x1) - x(d.x0) -1 ; })
          .attr("height", function(d) { return height - y(d.length); })
          .style("fill", "#69b3a2")


    // If less bar in the new histogram, I delete the ones not in use anymore
    u
        .exit()
        .remove()

    }


  // Initialize with 20 bins
  update(20)


  // Listen to the button -> update if user change it
  d3.select("#nBin").on("input", function() {
    update(+this.value);
  });

});
</script>

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