Most basic histogram in d3.js




This post describes how to build a very basic histogram with d3.js. Only one category is represented, to simplify the code as much as possible. You can see many other examples in the histogram section of the gallery. Learn more about the theory of histogram in data-to-viz.com.


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>


<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));

  // 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(70)); // then the numbers of bins

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

  // Y axis: scale and draw:
  var y = d3.scaleLinear()
      .range([height, 0]);
      y.domain([0, d3.max(bins, function(d) { return d.length; })]);   // d3.hist has to be called before the Y axis obviously
  svg.append("g")
      .call(d3.axisLeft(y));

  // append the bar rectangles to the svg element
  svg.selectAll("rect")
      .data(bins)
      .enter()
      .append("rect")
        .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")

});
</script>

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