# Teach Yourself D3.js

D3 is a JavaScript framework used for data visualization.
Learning how to work with D3.js is very easy. If you begin with the basics, you’ll see how easy it is to use D3, because:

• the DOM function names are much shorter than those in the legacy DOM library.
• the same functions are used for both setting and getting values.
• setters return a value, so you can keep working with the same element without repeating its name.

If you already know some framework, you probably feel that there’s nothing new under the sun, which is good because it makes learning the new stuff easier.

## So, What’s New?

In D3, you can:
* add and remove SVG and HTML elements using the setters and getter.
* set and get the element’s properties, attributes, styles, text and… data

## Getting Started

After you include it in your script, you can use selections, that is wrappers for elements.
To get a selection, use one of the functions:

• d3.select(selector) – to select the first element matching selector.
• d3.selectAll(selector) – to select the all elements matching selector.

Like the d3 object, selections has their select and selectAll methods to select descendant.

If selection is a selection, it’s easy to understand what selection.text(), selection.attribute(), selection.style(), selection.insert(), and selection.append() do. But, what does selection.data() do?

## Data And Update Selection

The selection’s method data() helps you use the HTML DOM as a database.
selection.data() returns an array of all data that belongs to the elements of the selectionץ The data is held in the property __data__ of the element.
selection.data(arr, func) – defines an update selection and distribute the elements of array arr to the elements of selection.
func(d,i)is a function that:

• receives d an element of arr and its index i.
• returns a record key.

If func is not passed, the key is the element’s index.

A recommended way to create an update selection is:
var updSelection = selection1.selectAll(selector).data(arr)

Now, updSelection is a selection of existing elements whose data has been changed.
In addition, updSelection has to methods:

• updSelection.enter() – returns place holders for elements to be created under selection1.
• updSelection.exit()– returns elements that have lost their data, and are usually to be removed.

### Using the data

The methods attr, style, property, text and data can accept a function as a value to set. The function will be called for each element of the selection’s data. And its arguments will be the array element and its index.

Now, let us see it in an example.
The following code arranges data in a donut layout::

<!DOCTYPE html>
<html>
<meta http-equiv="Content-type" content="text/html; charset=utf-8">
<title>Testing Pie Chart</title>
<script type="text/javascript" src="d3.min.js"></script>

<style type="text/css">
.slice text {
font-size: 12pt;
font-family: Arial;
}
</style>
<body>
<script type="text/javascript">
var width = 300,
height = 300,
color = d3.scale.category20c(); //builtin range of colors

// Step 1: creating a container for the donut chart, and attach raw data to it.
var vis = d3.select("body")
.append("svg")     //create the SVG element inside the <body>
.datum([{"label":"first", "value":20},
{"label":"second", "value":10},
{"label":"third", "value":30},
{"label":"fourth", "value":25}])
.attr({"width": width, "height": height})
.append("g")
.attr("transform", "translate(" + radius + "," + radius + ")") //move the center of the pie chart from 0, 0 to r, r

// Step two: defining the accessors of the layour function.
//           The function 'pie' will use the 'value' accessor to compute the arc's length.
var pie = d3.layout.pie()
.value(function(d) { return d.value; })
.sort(function(a,b){return a.label<b.label?-1:a.label==b.label?0:1;});

// Step 3: Create the update selection.
.data(pie);    // This will use the function 'pie' to create arc data from the row data attached to the visualisation element.

// Step 4: Using the data in the update selection to create shape elements.
var arc = d3.svg.arc()
;
.append('g')
.attr('class','slice')
.append("path")
.style("fill", function(d, i) { return color(i); } ) //set the color for each slice to be chosen from the color function defined above
.attr("d", arc);                                     // Create the arc from the arc data attached to each element.

.attr("transform",
function(d) {
return "translate(" + arc.centroid(d) + ")";
})
.attr("text-anchor", "middle")
.text(function(d, i) { return d.data.label; });

</script>
</body>
</html> 

And following is the chart:

For more funcions, see the API Reference.

# The Proof That 2=4

Let’s look at the following equations:
1. $x^{x^{x^{ \ldots}}} = 2$
2. $y^{y^{y^{\ldots}}} = 4$

From (1) we get that $x^2 = 2$, thus $x=\sqrt 2$

From (2) we get that $y^4 = 4$, thus $y = \sqrt[4]{4}$

But $\sqrt 2 = \sqrt[4]4$

And thus …
$2 = x^{x^{x^{\ldots}}} = \sqrt 2^{\sqrt 2^{\sqrt 2 ^ {\ldots}}} = \sqrt[4]4^{\sqrt[4]4^{\sqrt[4]4^ {\ldots}}} = y^{y^{y^{\ldots}}} = 4$

QED