# Creating a Pie Chart
## Set Up
```html
```
## Set Config Vars
```javascript
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.
```javascript
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:
```javascript
console.log(mapper.invert(45));
```
## Create the color scale to map labels to colors
D3 comes with lots of pre-made color schemes:
- https://github.com/d3/d3-scale/blob/master/README.md#category-scales
- https://github.com/d3/d3-scale-chromatic#categorical
They're just arrays:
```javascript
console.log(d3.schemeCategory10)
```
Consequently, we can use them when setting a range:
```javascript
var colorScale = d3.scaleOrdinal();
colorScale.range(d3.schemeCategory10);
```
We can generate an array of labels for the domain using JavaScript's map function:
```javascript
colorScale.domain(dataset.map(function(element){
return element.label;
}));
```
## Set up the svg
Standard:
```javascript
d3.select('svg')
.attr('width', WIDTH)
.attr('height', HEIGHT);
```
## Add paths for each pie segment
```javascript
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:
```javascript
.attr('d', function(datum){
//return path string here
})
```
Fortunately, D3 can generate something like this for us:
```javascript
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:
```javascript
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
```javascript
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
```javascript
console.log(pie(dataset));
```
We can use this when attaching data to our paths:
```javascript
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
```javascript
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:
```javascript
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:
```javascript
var arc = d3.arc()
.innerRadius(100) //to make this a donut graph, adjust this value
.outerRadius(radius);
```