# D3¶

D3 has several methods that let you add and change elements in your document.

The select() method selects one element from the document. It takes an argument for the name of the element you want and returns an HTML node for the first element in the document that matches the name. Here's an example:

const anchor = d3.select("a");

The above example finds the first anchor tag on the page and saves an HTML node for it in the variable anchor. You can use the selection with other methods. The "d3" part of the example is a reference to the D3 object, which is how you access D3 methods.

Two other useful methods are append() and text().

The append() method takes an argument for the element you want to add to the document. It appends an HTML node to a selected item, and returns a handle to that node.

The text() method either sets the text of the selected node, or gets the current text. To set the value, you pass a string as an argument inside the parentheses of the method.

Here's an example that selects an unordered list, appends a list item, and adds text:

d3.select("ul")
.append("li")
.text("Very important item");


D3 allows you to chain several methods together with periods to perform a number of actions in a row.

## Select All¶

D3 also has the selectAll() method to select a group of elements. It returns an array of HTML nodes for all the items in the document that match the input string. Here's an example to select all the anchor tags in a document:

const anchors = d3.selectAll("a");

Like the select() method, selectAll() supports method chaining, and you can use it with other methods.

## Work with data¶

The D3 library focuses on a data-driven approach. When you have a set of data, you can apply D3 methods to display it on the page. Data comes in many formats, but this challenge uses a simple array of numbers.

The first step is to make D3 aware of the data. The data() method is used on a selection of DOM elements to attach the data to those elements. The data set is passed as an argument to the method.

A common workflow pattern is to create a new element in the document for each piece of data in the set. D3 has the enter() method for this purpose.

When enter() is combined with the data() method, it looks at the selected elements from the page and compares them to the number of data items in the set. If there are fewer elements than data items, it creates the missing elements.

Here is an example that selects a ul element and creates a new list item based on the number of entries in the array:

<body><ul></ul><script>
const dataset = [:fontawesome-solid-link: "a", "b", "c"];
d3.select("ul").selectAll("li")
.data(dataset)
.enter()
.append("li")
.text("New item");
</script></body>


It may seem confusing to select elements that don't exist yet. This code is telling D3 to first select the ul on the page. Next, select all list items, which returns an empty selection. Then the data() method reviews the dataset and runs the following code three times, once for each item in the array. The enter() method sees there are no li elements on the page, but it needs 3 (one for each piece of data in dataset). New li elements are appended to the ul and have the text "New item".

### Work with dynamic data¶

The last two challenges cover the basics of displaying data dynamically with D3 using the data() and enter() methods. These methods take a data set and, together with the append() method, create a new DOM element for each entry in the data set.

In the previous challenge, you created a new h2 element for each item in the dataset array, but they all contained the same text, "New Title". This is because you have not made use of the data that is bound to each of the h2 elements.

The D3 text() method can take a string or a callback function as an argument:

selection.text((d) => d)

In the example above, the parameter d refers to a single entry in the dataset that a selection is bound to.

Using the current example as context, the first h2 element is bound to 12, the second h2 element is bound to 31, the third h2 element is bound to 22, and so on.

const dataset = [:fontawesome-solid-link: 12, 31, 22, 17, 25, 18, 29, 14, 9];

d3.select("body").selectAll("h2")
.data(dataset)
.enter()
.append("h2")
.text(d => ${d} USD);  ## Inline styling¶ D3 lets you add inline CSS styles on dynamic elements with the style() method. The style() method takes a comma-separated key-value pair as an argument. Here's an example to set the selection's text color to blue: selection.style("color","blue"); ## Dynamic styling¶ D3 is about visualization and presentation of data. It's likely you'll want to change the styling of elements based on the data. You can use a callback function in the style() method to change the styling for different elements. For example, you may want to color a data point blue if it has a value less than 20, and red otherwise. You can use a callback function in the style() method and include the conditional logic. The callback function uses the d parameter to represent the data point: selection.style("color", (d) => { /* Logic that returns the color based on a condition */ });  The style() method is not limited to setting the color - it can be used with other CSS properties as well. const dataset = [:fontawesome-solid-link: 12, 31, 22, 17, 25, 18, 29, 14, 9]; d3.select("body").selectAll("h2") .data(dataset) .enter() .append("h2") .text((d) => (d + " USD")) .style("color", (d) => d<20?"red":"green")  ## Add classes with D3¶ Using a lot of inline styles on HTML elements gets hard to manage, even for smaller apps. It's easier to add a class to elements and style that class one time using CSS rules. D3 has the attr() method to add any HTML attribute to an element, including a class name. The attr() method works the same way that style() does. It takes comma-separated values, and can use a callback function. Here's an example to add a class of "container" to a selection: selection.attr("class", "container"); Note that the "class" parameter will remain the same whenever you need to add a class and only the "container" parameter will change. <style> .bar { width: 25px; height: 100px; display: inline-block; background-color: blue; } </style> <body> <script> const dataset = [:fontawesome-solid-link: 12, 31, 22, 17, 25, 18, 29, 14, 9]; d3.select("body").selectAll("div") .data(dataset) .enter() .append("div") .attr("class", "bar") </script> </body>  ## Update height dynamically¶ The previous challenges covered how to display data from an array and how to add CSS classes. You can combine these lessons to create a simple bar chart. There are two steps to this: 1) Create a div for each data point in the array 2) Give each div a dynamic height, using a callback function in the style() method that sets height equal to the data value Recall the format to set a style using a callback function: selection.style("cssProperty", (d) => d) <style> .bar { width: 25px; height: 100px; display: inline-block; background-color: blue; } </style> <body> <script> const dataset = [:fontawesome-solid-link: 12, 31, 22, 17, 25, 18, 29, 14, 9]; d3.select("body").selectAll("div") .data(dataset) .enter() .append("div") .attr("class", "bar") .style("height", (d) => ${d}px);
</script>
</body>


## SVG¶

SVG stands for Scalable Vector Graphics.

Here "scalable" means that, if you zoom in or out on an object, it would not appear pixelated. It scales with the display system, whether it's on a small mobile screen or a large TV monitor.

SVG is used to make common geometric shapes. Since D3 maps data into a visual representation, it uses SVG to create the shapes for the visualization. SVG shapes for a web page must go within an HTML svg tag.

CSS can be scalable when styles use relative units (such as vhvw, or percentages), but using SVG is more flexible to build data visualizations.

Add an svg node to the body using append(). Give it a width attribute set to the provided w constant and a height attribute set to the provided h constant using the attr() or style() methods for each. You'll see it in the output because there's a background-color of pink applied to it in the style tag.

NoteWhen using attr() width and height attributes do not have units. This is the building block of scaling - the element will always have a 5:1 width to height ratio, no matter what the zoom level is.

<style>
svg {
background-color: pink;
}
</style>
<body>
<script>
const dataset = [:fontawesome-solid-link: 12, 31, 22, 17, 25, 18, 29, 14, 9];

const w = 500;
const h = 100;

const svg = d3.select("body")
.append("svg")
.attr("width", w)
.attr("height", h)
</script>
</body>


## Display shapes with svg¶

The last challenge created an svg element with a given width and height, which was visible because it had a background-color applied to it in the style tag. The code made space for the given width and height.

The next step is to create a shape to put in the svg area. There are a number of supported shapes in SVG, such as rectangles and circles. They are used to display data. For example, a rectangle (<rect>) SVG shape could create a bar in a bar chart.

When you place a shape into the svg area, you can specify where it goes with x and y coordinates. The origin point of (0, 0) is in the upper-left corner. Positive values for x push the shape to the right, and positive values for y push the shape down from the origin point.

To place a shape in the middle of the 500 (width) x 100 (height) svg from last challenge, the x coordinate would be 250 and the y coordinate would be 50.

An SVG rect has four attributes. There are the x and y coordinates for where it is placed in the svg area. It also has a height and width to specify the size.

Add a rect shape to the svg using append(), and give it a width attribute of 25 and height attribute of 100. Also, give the rect x and y attributes each set to 0.

<body>
<script>
const dataset = [:fontawesome-solid-link: 12, 31, 22, 17, 25, 18, 29, 14, 9];

const w = 500;
const h = 100;

const svg = d3.select("body")
.append("svg")
.attr("width", w)
.attr("height", h)
.append("rect")
.attr("width", 25)
.attr("height",100)
.attr("x",0)
.attr("y",0);
</script>
</body>


## Bar¶

<body>
<script>
const dataset = [:fontawesome-solid-link: 12, 31, 22, 17, 25, 18, 29, 14, 9];

const w = 500;
const h = 100;

const svg = d3.select("body")
.append("svg")
.attr("width", w)
.attr("height", h);

svg.selectAll("rect")
.data(dataset)
.enter()
.append("rect")
.attr("x", (d, i) => i * 30)
.attr("y", (d, i) => h - 3 * d)
.attr("width", 25)
.attr("height", (d, i) => 3 * d)
.attr("fill", "navy");

svg.selectAll("text")
.data(dataset)
.enter()
.append("text")
.attr("x", (d,i)=>i*30)
.attr("y", (d,i)=>h-3*d -3)
.text(d=>d);
</script>
<body>