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D3 Build

Lesson Objectives

Add link to d3 library
Add an <svg> tag and size it with D3
Create some fake data for our app
Add SVG circles and style them
Create a linear scale
Attach data to visual elements
Use data attached to a visual element to affect its appearance

Add link to d3 library

First thing we want to do is create basic index.html file:

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title></title>
    </head>
    <body>
    </body>
</html>

Now add a link to D3 at the bottom of your <body> tag in index.html:

<body>    
    <script src="https://d3js.org/d3.v4.min.js"></script>
</body>

Now create app.js, which will store all of our code:

console.log('this works');

and link to it in index.html at the bottom of the <body> tag:

<body>
    <script src="https://d3js.org/d3.v4.min.js"></script>
    <script src="app.js" charset="utf-8"></script>
</body>

Add an `<svg>` tag and size it with D3

At the top of the <body> tag in index.html, add an <svg> tag:

<body>
    <svg></svg>
    <script src="https://d3js.org/d3.v4.min.js"></script>
    <script src="app.js" charset="utf-8"></script>
</body>

In app.js create variables to hold the width and height of the <svg> tag:

var WIDTH = 800;
var HEIGHT = 600;

Next, we can use d3.select() to select a single element, in this case, the <svg> element:

var WIDTH = 800;
var HEIGHT = 600;

d3.select('svg');

The return value of this is a d3 version of the element (just like jQuery), so we "chain" commands onto this. Let's add some styling to adjust the height/width of the element:

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

Create some fake data for our app

In app.js let's create an array of "run" objects (NOTE I'm storing the date as a string on purpose):

var WIDTH = 800;
var HEIGHT = 600;

var runs = [
    {
        id: 1,
        date: 'October 1, 2017 at 4:00PM',
        distance: 5.2
    },
    {
        id: 2,
        date: 'October 2, 2017 at 5:00PM',
        distance: 7.0725
    },
    {
        id: 3,
        date: 'October 3, 2017 at 6:00PM',
        distance: 8.7
    }
];

Add SVG circles and style them

Add three circles to your <svg> element (each one will represent a run):

<svg>
    <circle/>
    <circle/>
    <circle/>
</svg>

Create app.css with some styling for the circles and our svg element:

circle {
    r:5;
    fill: black;
}
svg {
    border: 1px solid black;
}

and link to it in index.html

<head>
    <meta charset="utf-8">
    <title></title>
    <link rel="stylesheet" href="app.css">
</head>

Create a linear scale

One of the most important things that D3 does is provide the ability to map points in the "domain" of data to points in the visual "range" using what's called a scale.
There are lots of different kinds of scales, but for now we're just going to use a linear scale which will map numeric data values to numeric visual values.

In app.js:

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

var yScale = d3.scaleLinear(); //create the scale
yScale.range([HEIGHT, 0]); //set the visual range (e.g. 600 to 0)
yScale.domain([0, 10]); //set the data domain (e.g. 0 to 10)

console.log(yScale(5)); //get a visual point from a data value
console.log(yScale.invert(450)); //get a data values from a visual point

Here we're saying that a data point of 0 to map to a visual height value of 600
This is because the lower the distance run (data value), the more we want to move the visual point down the Y axis
- remember that the Y axis starts at 0 at the top and increases in value

Attach data to visual elements

We can attach each of our "run" objects to one of our circles, so that each circle can access that data:

yScale.range([HEIGHT, 0]);
yScale.domain([0, 10]);

d3.selectAll('circle').data(runs); //selectAll is like select, but selects all elements that match the query string

Use data attached to a visual element to affect its appearance

When setting a value for an element's style, class, id or any other attribute, we can pass that method a callback instead of a static value.

d3.selectAll('circle').data(runs)
    .attr('cy', function(datum, index){
        return yScale(datum.distance);
    });

That callback function runs for each visual element selected
The result of the function is then assigned to whatever aspect of the element is being set (in this case the cy attribute)
The callback function takes two params
- the individual datum attached to that particular visual element
- the index of that datum in the original data array (in this case runs)

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