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Creating an Animated Ring or Pie chart in d3js

  In our previous article, we learnt how to create a simple animated bar graph. In this article, we will tackle something thats a little more interesting - creating a an animated ring. We will follow a step-by-step process to create a simple animated ring whose contents change based upon the options you select from a dropdown. DEMO TLDR: These are the major steps towards achieving our goal

Data and Dimentions

We begin by creating some variables that will be used by our drawAnimatedRingChart function.

// Data: Average age of employees in an organization
var employees = [
    {dept: 'A', count : 22},
    {dept: 'B', count : 66},
    {dept: 'C', count : 25},
    {dept: 'D', count : 50},
    {dept: 'E', count : 27}
];
var maxWidth = 200;
var maxHeight = 200;
var outerRadius = 100;
var ringWidth = 20;

D3.layout.pie

The main point to remember here is that a ring in d3 is just a variation of the pie chart with the following difference -

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While a pie chart only requires an outer radius (the inner radius defaulting to 0), a ring has a non-zero value for both - outer radius and inner radius.

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That said, lets start defining our function responsible for drawing the ring. In the above snippet we created a pie function that can extracts the 'count' property of a data array and return a corresponding array of objects with each object having two new attributes - the startAngle and the endAngle. This new array can then be consumed by another function to draw a section of the pie - the arc function. We need two more functions that will help us draw the actual pie - one to define the colors of our pie chart, the other is our arc function. d3.scale.catgory10 is a convenience function that creates an array of 10 colors. d3.svg.arc is the meaty function here. The arc function understands the output array of d3.svg.pie and and can draw an arc shape based upon the startAngle and endAngle of the objects in that array. Since we want to draw a ring, we will need to set both - the outer and inner radius provided in the arguments. If you remember the example above, since we we will draw different rings based upon the item chosen in the dropdown, its important to clear the svg of its previous contents before attempting a redraw.

// Remove the previous ring
d3.select(config.el).selectAll('g').remove();
var svg = d3.select(config.el)
    .attr({
        width : maxWidth,
        height: maxHeight
    });

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Drawing the arc

With some initial setup out of the way, its a great time to start drawing our ring. We first create a group for each arc to be drawn. Notice that we used pie(config.data) when binding our data. We did not directly associate our raw data array employees with the groups, instead we transformed it such that each data point that is bound to the DOM will now posess a startAngle and an endAngle - thanks to the pie function[ Tip: Learn how data binding works in d3 ]. In the next step, we draw the pie and assign it some color. And with the above snippet, we are finally able to draw a ring, albeit without animation. Notice that we created a path with an attribute called d. Thats because in order to draw the actual section of a ring, the startAngle and endAngle attributes need to be transformed into svg coordinates in a way that a path can be drawn around it and that task is performed by the pie function. JSFIDDLE DEMO

Animating the ring

In order to create the animation, we will need tweak our code a bit. First we need to create a function that knows how to generate different values of the end angle for each arc over a period of time. In d3, such functions that generate time dependent values are called interpolators. In the above code, we created an interpolator function i that can generate intermediate values between a start and finish object. The arc function then consumes these intermediate values to render the state of the arc over a period of time, thereby causing an animated effect. Here's how you'd use the tweenPie function. There are three important points to notice here.

• We invoked a transition function on our selector.
• We specified a duration of the transition.
• Instead of using attr we now use attrTween and pass it our tweenPie function defined earlier

JSFIDDLE DEMO That pretty much achieves what we set out to do. Now lets give it a final touch.

Adding labels

Adding our label to a chart is pretty easy. All you would need to do is add the following snippet. We just translated the text to the center of each ring section and then set the text. Notice that we had to use d.data because if you remember from earlier d in our case was the output of the pie function. There is however one problem with the above snippet. The labels dont seem to show up. The reason is that due to the animation in the ring, it gets drawn on top of the labels. To get this working, we will need to draw the lables after the entire animation is complete. In D3, As of this writing the end event for a transition is fired for each animated item in the selection. So the best way to know when the entire animation is complete is through reference counting. So, we are going to define a little callback function outside of our ring function that we will then bind to the end event of every function to do the counting for us. And thats how you get the labels to appear on top of the UI after the animation ends. FINAL JSFIDDLE DEMO

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And thats all there is to it. Found something incorrect or want to see something improved? Let us know in the comments.

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Also Read:

1. d3 Fundamentals : Understanding domain, range and scales in D3js
2. Selecting a node in d3js based upon the value of its data
3. A visual explanation of the Enter, Update and Exit Selections in D3js
4. d3 Fundamentals: Creating a simple bar graph

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