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Creating a Pie Chart

Set Up

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title></title>
        <script src="https://d3js.org/d3.v4.min.js"></script>
    </head>
    <body>
        <svg>
            <g></g>
        </svg>
        <script src="app.js" charset="utf-8"></script>
    </body>
</html>

Set Config Vars

var WIDTH = 360;
var HEIGHT = 360;
var radius = Math.min(WIDTH, HEIGHT) / 2;

var dataset = [
    { label: 'Abulia', count: 10 },
    { label: 'Betelgeuse', count: 20 },
    { label: 'Cantaloupe', count: 30 },
    { label: 'Dijkstra', count: 40 }
];

Create an Ordinal Scale

An ordinal scale maps discrete values (can't be interpolated) to discrete values.

var mapper = d3.scaleOrdinal();
mapper.range([45, 63, 400]); //list each value for ordinal scales, not min/max
mapper.domain(['Bob', 'Sally', 'Zagthor']); //list each value for ordinal scales, not min/max

console.log(mapper('Bob'));
console.log(mapper('Sally'));
console.log(mapper('Zagthor'));

You cannot invert ordinal scales:

console.log(mapper.invert(45));

Create the color scale to map labels to colors

D3 comes with lots of pre-made color schemes:

They're just arrays:

console.log(d3.schemeCategory10)

Consequently, we can use them when setting a range:

var colorScale = d3.scaleOrdinal();
colorScale.range(d3.schemeCategory10);

We can generate an array of labels for the domain using JavaScript's map function:

colorScale.domain(dataset.map(function(element){
    return element.label;
}));

Set up the svg

Standard:

d3.select('svg')
    .attr('width', WIDTH)
    .attr('height', HEIGHT);

Add paths for each pie segment

var path = d3.select('g').selectAll('path')
    .data(dataset)
    .enter()
    .append('path')
    .attr('fill', function(d) {
        return colorScale(d.label);
    });

Generate an arc creating function

Next we want to do something like this:

.attr('d', function(datum){
    //return path string here
})

Fortunately, D3 can generate something like this for us:

var arc = d3.arc()
    .innerRadius(0) //to make this a donut graph, adjust this value
    .outerRadius(radius);

We could plug this function into it's right place, but it won't work yet:

var path = d3.select('g').selectAll('path')
    .data(dataset)
    .enter()
    .append('path')
    .attr('d', arc) //add this
    .attr('fill', function(d) {
        return colorScale(d.label);
    });

Format the data for the arc

The reason that our arc() function won't work is that the data isn't formatted properly for the function
The arc function we generated expects the data object to have things like start angle, end angle, etc
D3 can reformat our data so that it will work with our generated arc() function

var pie = d3.pie()
    .value(function(d) { return d.count; }) //use the 'count' property each value in the original array to determine how big the piece of pie should be
    .sort(null); //don't sort the values

pie is a function that takes an array of values as a param and returns an array of objects that are formatted for our arc function

console.log(pie(dataset));

We can use this when attaching data to our paths:

var path = d3.select('g').selectAll('path')
    .data(pie(dataset)) //reformat data for arc
    .enter()
    .append('path')
    .attr('d', arc)
    .attr('fill', function(d) {
        return colorScale(d.label);
    });

Unfortunately, now each object from the data array that's been attached to our path elements doesn't have a .label property
Fortunately, it does have a .data attribute that mirrors what the data looked like before we passed it to the pie() function

var path = d3.select('g').selectAll('path')
    .data(pie(dataset))
    .enter()
    .append('path')
    .attr('d', arc)
    .attr('fill', function(d) {
        return colorScale(d.data.label); //use .data property to access original data
    });

Adjust the position of the pie

The pie starts at (0,0), so we can move the group containing the pie:

var container = d3.select('g')
    .attr('transform', 'translate(' + (WIDTH / 2) + ',' + (HEIGHT / 2) + ')'); //pie center is at 0,0

Make a donut graph

If you want the pie to have a hole at the center, just adjust the inner radius of the arc() function:

var arc = d3.arc()
    .innerRadius(100) //to make this a donut graph, adjust this value
    .outerRadius(radius);

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