A "simple" D3.js area chart

By Matt Layman on January 24, 2015

Let’s take:

XY
010
115
235
320

And make:

D3.js is mindbending, and I find the examples on the D3.js wiki to be too little explanation with too much going on. In this example, I will show you how to make the simplest area chart I could devise. If you want to jump straight to “the answer,” see the complete JavaScript.

D3.js is not a chart library. It is a chart parts library. The library feels like a mashup of SVG and data manipulation with some sugar sprinkled on top. While immensely flexible, the flexibility comes at the cost of complexity. Let’s dive in.

To build the chart, we need: data, an SVG container, margins, an X axis, a Y axis, the area shape itself, and some CSS to make it look pretty.

Data

We’re not going to mess with TSV or CSV loaders or any of the callback stuff. Here is the data, plain and simple.

var data = [
    { x: 0, y: 10, },
    { x: 1, y: 15, },
    { x: 2, y: 35, },
    { x: 3, y: 20, },
];

SVG

D3 uses SVG (Scalable Vector Graphics) to draw its shapes. It’s possible to create a new <svg> tag on the fly, but I added the following to the HTML source code.

<svg id="area" />

Margins

Charts in D3 have no margins, but the primary D3 author frequently talks about defining conventional margins. The idea is to make some margins and define an SVG group (i.e. the g tag) that is set to those margin boundaries. The code only considers the group tag as the drawable area.

var margin = {top: 20, right: 20, bottom: 40, left: 50},
    width = 575 - margin.left - margin.right,
    height = 350 - margin.top - margin.bottom;

Axes

To draw data in a scalable way, D3 needs to be able to map the data (e.g., x=0, y=10) to a pixel position. We must take the X data and set it on the axis so that the maximum X value (i.e. 3) matches the pixel width of the chart area. Because D3 is so flexible, it means that X and Y must be defined independently.

In math class, you were probably taught that X is for the domain and Y is for the range. Unfortunately, D3 uses domain/range terms to apply to axes too. We have to think about the X data (0 - 3) as the domain, and the chart horizontal dimension (0 - width) as the range. The same kind of thinking has to be applied for the Y axis as well (0 - 35 applied to the chart vertical dimension).

You can think of the x and y variables as translator functions that take a domain value and convert it to a pixel size. xAxis and yAxis are indicating where the axes should go.

var x = d3.scale.linear()
    .domain([0, d3.max(data, function(d) { return d.x; })])
    .range([0, width]);

var y = d3.scale.linear()
    .domain([0, d3.max(data, function(d) { return d.y; })])
    .range([height, 0]);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left");

Area

The area function transforms each data point like (2, 35) into information that describes the shape. Each corresponds to an x position, an upper y position, y1, and a lower y position, y0. The odd thing here is that y0 is set to the constant of height. This makes sense when you know that SVGs are positioned relative to the upper left corner of the graphic. Any distance “down” is a positive number, so a positive height means the bottom of the graphic.

var area = d3.svg.area()
    .x(function(d) { return x(d.x); })
    .y0(height)
    .y1(function(d) { return y(d.y); });

Putting it all together

So far, we have not done anything except define some data and functions. Now we need to put those functions to work.

var svg = d3.select("svg#area")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

svg.append("path")
    .datum(data)
    .attr("class", "area")
    .attr("d", area);

svg.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")
    .call(xAxis);

svg.append("g")
    .attr("class", "y axis")
    .call(yAxis);

The variable definition of svg grabs the svg tag with ID area and adds a group tag, g, to define the margins within the SVG. All the drawing will happen inside this g tag.

The next section adds a path. This is where the data and the area function meet. It is the keystone of this entire example. D3 uses each data point and passes it to the area function. The area function translates the data into positions on the path in the SVG. It will result in:

<path class="area" d="M0,214.28571428571428L168.33333333333331,
171.42857142857142L336.66666666666663,0L505,128.57142857142858L505,
300L336.66666666666663,300L168.33333333333331,300L0,300Z"/>

The last two sections adds the axes to the SVG. There isn’t much to say about them.

Make it pretty

In “Putting it all together”, I ignored the .attr("class", "area") explanations. D3 can add any tag attributes with attr(). I added some class attributes so that the graph can be styled. SVG uses different properties than standard HTML tags, but the styling below gave the graph its simple look.

svg { border: 1px solid #dedede; }

.axis path, .axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.area { fill: #4ca3bd; }

Other charts

In this post, I covered the basics of making an area chart. In future posts, I will explain how to use other types of data and how to make stacked area charts. Stay tuned!

If you want to chat about this with me, I'm @mblayman on Twitter.



Matt Layman

Matt is the lead software engineer at Storybird.

Always eager to talk about Python and other technology topics, Matt organizes Python Frederick in Frederick, Maryland (NW of Washington D.C.) and seeks to grow software skills for people in his community.