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Short D3 presentation for the IT Dept. at UiT
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This is a small guide/presentation about d3 that I did for the IT. Department's Developer lunch 29. October 2014. To follow the guide, clone down the repo and start a simple HTTP server in that directory, e.g.

git clone
cd d3-presentation
python -m SimpleHTTPServer

Then head over there and navigate through this guide. The different pages are named somewhat similar to the headers in the readme.

What is d3 and why you should care

D3 is a JavaScript library for manipulating documents based on data. With it you can bring any type of data to life in a modern web browser. It uses HTML, SVG and CSS putting an emphasis on web standards. You can use D3 to do virtually anything with your data. Create a table or make some fancy graphics with insane animations.

But be aware this is JavaScript we're dealing with! (For fun, try to enter [] + [] or [] + {} or {} + [] or {} + {} in a console in your browser. You'll be surprised of what it actually does!)


The first thing we're going to do is looking at how you typically would modify a web page through some javascript. First up, from an empty html page, create a div and insert some text into it.

Open up 1.html in a web browser and open the console (In Chrome on a mac you can open it by ⌥⌘J).

var div = document.createElement("div");
div.innerHTML = "Hello, world!";

Now with HTML we can create nice little graphics with SVGs. We can modify the contents of the div we just created with a circle!

div.innerHTML = '<svg width="100" height="100"> <circle cx="30" cy="60" r="10"></circle></svg>'

Cool right? Next up: D3.


To get started with D3, just add

<script src="" charset="utf-8"></script>

to the header of the HTML file.

Using d3 to do the same thing as we did.

var body ="body");
var div = body.append("div");
div.html("Hello, world!");

Another nice thing about selections is method chaining

var body ="body").append("div").html("Hello, mate!");

What is really cool though is that with D3 modifying the DOM is pretty easy. First let's set up some more divs and I'll show you the power of D3.

for (var i = 0; i < 10; i++){
    var div = document.createElement("div");
    div.innerHTML = "Hello " + i;

Normally, if we would like to make the text in these divs turn red, we would type something like:

var divs = document.getElementsByTagName("div");
for (j = 0; j < divs.length; j++){
    var div = divs.item(j)"color", "red", null);

In D3 we have something that's called selections that can select and operate on arbitrary sets of elements. Let's turn the text blue.


Now, we'll go some more into the details of how selections work.

Data Joins & Selections

Before we continue I think we should have a look at how data joins and selections work. As you can see the web page contains a body with 4 div's. D3 is really about mapping data to something in the DOM, divs, svgs, anything. Selections are really just groups of arrays of things. Lets try to create some data that we can join with these divs:

In the console type

var div = d3.selectAll("div");

to get a selection containing all of the 4 divs on our page. Check it by inspecting the object it returned. Now, if we want to join some data to these elements, we need a dataset

var dataset = [4,5,8,13]

Since we have got 4 items in the dataset array and 4 divs, each item should match its own div. Lets go ahead and join the the elements (divs) with data:

var div = d3.selectAll("div").data(dataset);

In D3 when you're joining elements to data by a key (in our case the index in the array will be the key) there are three outcomes:

  • Update. There was a matching element for a given datum (what we got now, every element correspond to a datum)
  • Enter. There was no matching element for a given datum (if we had 4 div elements, but 5 items in the dataset array, maybe we should create another element for this datum?).
  • Exit. There was no matching datum for a given element (if we had 4 divs but our dataset array only contained 3 items, could possibly remove a div?)

These are returned by div, div.enter() and div.exit() respectively. Have a look at them in the console. You should find that div contains a group of 4 divs, and div.enter() and div.exit() are both empty.

If you try to remove the last item from the dataset and join it again, you'll notice change in the div div.exit() selections.

dataset = [4,5,8]
div = d3.selectAll("div").data(dataset);

.selectAll - Wat

Ok, so so far we have modified the DOM that was set up for us. What if we have a completely empty HTML page and want to write some numbers to it? Lets create some numbers and write them out in separate paragraphs:

var dataset = [4, 8, 15, 16, 23, 42];
var p ="body").selectAll("p")

But wait, if the web page was empty, why do we use selectAll if we know the paragraphs don't exist? With D3, you don't tell it how you do something, but what you want it to do. We want the numbers in the dataset to correspond to paragraphs.

If we now have a look at p.enter(), p and p.exit() we'll notice that only p.enter() has got any items. This is because there are no matching elements for any of the datum.

To make elements, we simply get the enter selection and create a <p> for each of them!

        return "Hi, I'm number " + d;


Let's try to manipulate some circles using selections. On the page we've got three pretty svg circles. Have a look at the source, to see how that looks in HTML.

Using selectAll we can get all of the circles.

var circle = d3.selectAll("circle");

With this selection, we can go ahead and change their color and radius."fill", "steelblue");
circle.attr("r", 30);

Using anonymous functions we can set values on a per-element basis. This function is evaluated for every element. Notice that every one of them have a cx attribute, this is the x coordinate of the centers of the circles. Using an anonymous function we can set the circle's x-coordinate.

circle.attr("cx", function() { 
    return Math.random() * 720;

Ok, but now, lets try to join some data to these circles. Refresh the page so that the circles are back where they started.

Like we did previously we join a dataset with the circles.

var circle = d3.selectAll("circle")[20, 40, 60])

With the dataset joined, we can set their radius according to bound data. Usually we use the name d but use whatever you want.

circle.attr("r", function(d){
    return d;

Now we got them all tangled up, let's space them a bit out. Using a second argument to the function we can get its index in the selection.

circle.attr("cx", function(d,i){
    return i * 100 + 30;

But what if we had more than 3 data elements? We would need a new circle!

Let's join the circles with some other data, now 4 numbers.

var circle ="svg")
                .data([20, 40, 60, 80]);

Have a look at what the circle variable looks like. D3 has joined our data with the circles on the page. Three of the data items are joined with a circle on the web page, but the last number (80) hasn't got a circle yet. Remember that every data item that is missing an element is placed in the enter selection.

So for every item that is not bound to an element, we append a <circle> to the svg:

var circleEnter = circle.enter().append("circle");

Have a look at the DOM tree to see that we've got a new circle.

With these four circles we can update their attributes, e.g. radius and location:

circle.attr("r", function(d){
    return d/2;

circle.attr("cy", 60);

circle.attr("cx", function(d,i){
    return i * 100 + 30;

If we updated our dataset to only contain the first two items, we would need to remove the circles without any data. Remember that the elements that do not correspond to data are placed in the exit selection.

var circle ="svg").selectAll("circle")

Putting all of that together

Putting it all together

var svg ="svg")

var circle = svg.selectAll("circle")


circle.attr("r", function(d){
          return d/2;
      .attr("cy", 60)
      .attr("cx", function(d,i){
          return i * 100 + 30;


Lets make a scatterplot!

var dataset = [{x:10, y:10},
                {x:20, y:80}];

var svg ="body")
            .attr("width", "300")
            .attr("height", "200");

var circle = svg.selectAll("circle")
      .attr("r", "4")
      .attr("cx", function(d) {
            return d.x
      .attr("cy", function(d) {
            return d.y

Moving on to a bar chart!

With D3 there are endless posibilities when it comes to visualizing your data. Let's try to make a bar chart.

First let's set up a dataset

var dataset = [2,3,7,11,13,17]

We want the bars to have a static width, and the <svg> element should be 400 pixels high.

var h = 400,
    barWidth = 30;

Then we set up the svg like we're used to.

var svg ="body")
                .attr("width", barWidth * dataset.length)
                .attr("height", h); 

Note that we set the width to be the number of items in the dataset multipled with the width of the bars. The bars will fit the svg perfectly.

We want to have the heights of the bars to fit the svg we have set up. Since we're dealing with small numbers in our dataset, we'll make use of d3.scale to scale them up to the size of the svg we are drawing with. D3 can help us map numbers in one domain, i.e. our dataset, to a new domain, i.e. pixels from 0 to the height of the svg.

var yScale = d3.scale.linear() 
                .domain([0, d3.max(dataset)])

This scale takes values [0,...,17] and maps them to values [0,...,h=400].

Next up is adding some bars. In our example we want to draw bars and write out the data element within the bar. With svg we have got many different elements we can chose from when we want to draw something. So far we've only had a look at <circle>, but there are many more!. Since we want both text and a shape, we choose the <g> element. This element is used to group objects, making it simple to place text and a shape in the same place.

Let's join the dataset with some <g> elements

var bar = svg.selectAll("g")
    .attr("transform", function(d,i){
        var yoffset = h-yScale(d)
        return "translate(" + i * barWidth + ","+ yoffset +")";

The first three lines should be familiar now. For any data that doesn't have a corresponding g element, create a new one. Then we add the transform attribute to move this g element to a new x, y location. This is because we want to start drawing the bars in different locations. The x coordinate will make sure that we place them one after another on the x-xis, and the y coordinate makes sure that the bars are drawn to look like they are growing upwards. Remember that the coordinate (0,0) is top left.

Next up is creating the shape that draws a rectangle. For this we have the svg element <rect>. We use the bar selection (with the g elements we created), and append a rectangle:

    .attr("height", function(d){
            return yScale(d);
    .attr("width", barWidth - 1)
    .style("fill", "#fab") 

The height is calculated from the data, width is the predefined width minus a pixel, and the fill color is a pretty shade of pink.

The last thing we need to get in there are labels for each bar. We use the same bar selection and append a text element:

    .attr("x", barWidth/2)
    .attr("y", 10) 
    .attr("dy", ".35em")
            return d;
    .attr("fill", "#5F5F5F") 
    .style("text-anchor", "middle")

The x attribute is in the middle of the rectagle, the y attribute is set to 10 (just a number that locates the label towards the top of the rectangle). Note that this coordinate is relative to the position of the g element. All of the attributes that were set for the g element is inherited by its children. In our case it's just the transform. The last 5 lines writes the text in a dark grey color, and makes sure that its centered within the g.


The last thing we're going to look at is how we can use D3 together with websockets to create a graph visualization that is updated from a server. Unfortunately I don't think we'll have time to look at absolutely everything today, so this will be a brief walkthrough of how it's done.

The idea of the whole thing is that we have some sort of server that keeps track of a graph (nodes and edges etc.). When the graph is updated it sends an updated list of nodes and edges to the client which can visualize the graph. The client visualizes the graph using D3 to represent nodes as circles and edges as lines. To make everything look pretty etc. ,we use d3.force to generate a force-directed layout.


The server is a small little thing written in go, looking very similar to Lars Tiede's websocket thing some weeks back. The only difference is that the server sends a graph rather than a number to the client. The graph is a just a struct that looks something like this when it's sent to the client:

Graph {
    Nodes: [
        {id: 1},
        {id: 3}
    Edges: [
        {source: 0, target:1},

Note that the source and target are indicies in the Nodes array. This is to make everything a bit more simple when we're working with d3.force. The naming is also to make things simpler.


We use D3 to join the data representation of a graph to a visual representation using <circle> and <line> svg elements. We can have a brief look at how the source code looks like:

    var width = 500,
        height = 500;

    var svg ="body").append("svg")
        .attr("width", width)
        .attr("height", height);

    var color = d3.scale.category20b();

    var force = d3.layout.force()
        .size([width, height]);

We create a new <svg> element with the given dimension. The color variable is a color scale that we'll use to color our nodes. It's categorical color scale with 20 different colors. For more info see Categorical Colors. Next up is the force variable, where we have set up our force directed layout. The charge and the linkDistance don't matter that much, see d3.force for more information.

Next is opening a websocket to the server. We use the same approach as Lars did:

    var ws = new WebSocket("ws://localhost:4040/graph");

    ws.onopen = function() {

    ws.onmessage = function(msg) {

Open a new websocket to the given URL and send an empty message to signal that we're up and runnig. When the client receives a message we should update the graph visualization according to the list of nodes and edges from the server.

Let's have a look at what happens within the ws.onmessage function:

        updatedGraph = $.parseJSON(

        graph = updateNodes(graph, updatedGraph.Nodes) 
        graph = updateEdges(graph, updatedGraph.Edges) 

We parse the JSON that we got from the server, so that we get a javascript object that we can work with. Usually we would just create <circle> elements based on the array of nodes that we received from the server. However, since we don't want to restart the force-directed layout every time we receive an updated list of nodes and edges, we need keep a local representation of the graph. This representation will contain the x and y locations for the <circle> and <link> elements as they are moved around.

We have some small helper functions updateNodes(graph, nodes) and updateEdges(graph, edges) that can help us keep track of nodes and edges, adding new ones as they are received. Both of these return a graph object that we can

Let's start up the force-directed layout with the list of nodes and edges


This will start to move nodes and edges around according to the force-directed layout we created. We're not drawing anything yet, so let's do that. We can start to join the nodes with <circle> elements.

        node = svg.selectAll(".node")

        nodeEnter = node.enter().append("circle")
                                .attr("class", "node")
                                .attr("r", 4) 
                                .style("fill", function(d){
                                      return color(;

Everything here should be familiar. Note that we use the color scale we set up earlier to color the node according to its id. The last line makes it possible to drag nodes around. For more information about how .call works, see Selections.

Next up is adding edges to the graph.

        edge = svg.selectAll(".link")
                  .data(graph.Edges, function(d) {
                      return + "-" +;

        edgeEnter = edge.enter().append("line")
                        .attr("class", "link")
                        .style("stroke-width", 2)
                        .style("stroke", "#999")
                        .style("stroke-opacity", ".6")

Everything here is pretty straight forward. The only thing here that's new is that we specify the key which D3 uses to map the data with the <line> element.

You might wonder where we've put these circles and links, but so far we have only created them not specified where to draw them. That's where the d3.force is going to help out. When we started the force-directed layout above it is modifying the arrays with our nodes and edges, modifying different attributes such as x and y, or source.x and target.x in the case of edges. To draw our elements in according to this values we can use force.on("tick", function)which is called on every step in the force simulation:

        force.on("tick", function() {
            edge.attr("x1", function(d) { return d.source.x; })
                .attr("y1", function(d) { return d.source.y; })
                .attr("x2", function(d) { return; })
                .attr("y2", function(d) { return; });

            node.attr("cx", function(d) { return d.x; })
                .attr("cy", function(d) { return d.y; });

Now we draw the nodes and edges where they're supposed to be!

If you want to see how everything is magically put together, head to the graphman directory. To run the server, run go run graphman.go. Then you can visit the graph visualization on localhost:4040.

That's it for this presentation, good job for following it to the end!

Helpful resources

This tutorial uses material from: Let's make a bar chart, Three little circles, Thinking with joins, How selections work and Lars Tiede's Utviklerlunsj.

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