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201 lines
4.7 KiB
201 lines
4.7 KiB
# Creating a Pie Chart
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## Set Up
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```html
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<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title></title>
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<script src="https://d3js.org/d3.v4.min.js"></script>
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</head>
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<body>
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<svg>
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<g></g>
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</svg>
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<script src="app.js" charset="utf-8"></script>
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</body>
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</html>
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```
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## Set Config Vars
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```javascript
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var WIDTH = 360;
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var HEIGHT = 360;
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var radius = Math.min(WIDTH, HEIGHT) / 2;
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var dataset = [
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{ label: 'Abulia', count: 10 },
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{ label: 'Betelgeuse', count: 20 },
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{ label: 'Cantaloupe', count: 30 },
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{ label: 'Dijkstra', count: 40 }
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];
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```
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## Create an Ordinal Scale
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An ordinal scale maps discrete values (can't be interpolated) to discrete values.
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```javascript
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var mapper = d3.scaleOrdinal();
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mapper.range([45, 63, 400]); //list each value for ordinal scales, not min/max
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mapper.domain(['Bob', 'Sally', 'Zagthor']); //list each value for ordinal scales, not min/max
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console.log(mapper('Bob'));
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console.log(mapper('Sally'));
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console.log(mapper('Zagthor'));
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```
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You cannot invert ordinal scales:
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```javascript
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console.log(mapper.invert(45));
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```
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## Create the color scale to map labels to colors
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D3 comes with lots of pre-made color schemes:
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- https://github.com/d3/d3-scale/blob/master/README.md#category-scales
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- https://github.com/d3/d3-scale-chromatic#categorical
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They're just arrays:
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```javascript
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console.log(d3.schemeCategory10)
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```
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Consequently, we can use them when setting a range:
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```javascript
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var colorScale = d3.scaleOrdinal();
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colorScale.range(d3.schemeCategory10);
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```
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We can generate an array of labels for the domain using JavaScript's map function:
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```javascript
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colorScale.domain(dataset.map(function(element){
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return element.label;
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}));
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```
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## Set up the svg
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Standard:
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```javascript
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d3.select('svg')
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.attr('width', WIDTH)
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.attr('height', HEIGHT);
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```
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## Add paths for each pie segment
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```javascript
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var path = d3.select('g').selectAll('path')
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.data(dataset)
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.enter()
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.append('path')
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.attr('fill', function(d) {
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return colorScale(d.label);
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});
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```
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## Generate an arc creating function
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Next we want to do something like this:
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```javascript
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.attr('d', function(datum){
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//return path string here
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})
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```
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Fortunately, D3 can generate something like this for us:
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```javascript
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var arc = d3.arc()
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.innerRadius(0) //to make this a donut graph, adjust this value
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.outerRadius(radius);
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```
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We could plug this function into it's right place, but it won't work yet:
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```javascript
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var path = d3.select('g').selectAll('path')
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.data(dataset)
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.enter()
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.append('path')
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.attr('d', arc) //add this
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.attr('fill', function(d) {
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return colorScale(d.label);
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});
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```
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## Format the data for the arc
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- The reason that our `arc()` function won't work is that the data isn't formatted properly for the function
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- The arc function we generated expects the data object to have things like start angle, end angle, etc
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- D3 can reformat our data so that it will work with our generated `arc()` function
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```javascript
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var pie = d3.pie()
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.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
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.sort(null); //don't sort the values
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```
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`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
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```javascript
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console.log(pie(dataset));
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```
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We can use this when attaching data to our paths:
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```javascript
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var path = d3.select('g').selectAll('path')
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.data(pie(dataset)) //reformat data for arc
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.enter()
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.append('path')
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.attr('d', arc)
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.attr('fill', function(d) {
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return colorScale(d.label);
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});
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```
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- Unfortunately, now each object from the data array that's been attached to our `path` elements doesn't have a `.label` property
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- Fortunately, it does have a `.data` attribute that mirrors what the data looked like before we passed it to the `pie()` function
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```javascript
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var path = d3.select('g').selectAll('path')
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.data(pie(dataset))
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.enter()
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.append('path')
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.attr('d', arc)
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.attr('fill', function(d) {
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return colorScale(d.data.label); //use .data property to access original data
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});
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```
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## Adjust the position of the pie
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The pie starts at (0,0), so we can move the group containing the pie:
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```javascript
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var container = d3.select('g')
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.attr('transform', 'translate(' + (WIDTH / 2) + ',' + (HEIGHT / 2) + ')'); //pie center is at 0,0
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```
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## Make a donut graph
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If you want the pie to have a hole at the center, just adjust the inner radius of the `arc()` function:
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```javascript
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var arc = d3.arc()
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.innerRadius(100) //to make this a donut graph, adjust this value
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.outerRadius(radius);
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```
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