D3.js

Context

The first web browsers appeared in the early 1990s. They were initially capable of displaying static web pages only: the only way for a user to interact with the web was through clicking links and scrolling pages. There were many efforts to overcome such limitations. One of the most significant was the integration of JavaScript as the scripting language for web browsers. JavaScript gradually became the de facto standard language for creating web pages with rich user interactivity. This played a crucial role in the decision to use JavaScript as the language of D3.js.

At the same time, researchers, engineers, and practitioners from various branches of engineering and science looked for tools that would enable web browsers to visually present data within web pages. There were multiple projects with that goal, each of which had its successes and failures, and inspired the subsequent ones. The most notable examples were the Prefuse, Flare, and Protovis toolkits, which can all be considered as direct predecessors of D3.js.

Prefuse was a visualization toolkit created in 2005 that required usage of Java, and visualizations were rendered within browsers with a Java plug-in. Flare was a similar toolkit from 2007 that used ActionScript, and required a Flash plug-in for rendering.

In 2009, based on the experience of developing and utilizing Prefuse and Flare, Professor Jeff Heer, Ph.D. student Mike Bostock, and M.S. student Vadim Ogievetsky of Stanford University's Stanford Visualization Group created Protovis, a JavaScript library to generate SVG graphics from data. The library received notable acceptance both by data visualization practitioners and academics.

In 2011, the development of Protovis was stopped to focus on a new project, D3.js. Informed by experiences with Protovis, Bostock, along with Heer and Ogievetsky, developed D3.js to provide a more expressive framework that, at the same time, focuses on web standards and provides improved performance.

Technical principles

Embedded within an HTML webpage, the JavaScript D3.js library uses pre-built JavaScript functions to select elements, create SVG objects, style them, or add transitions, dynamic effects or tooltips to them. These objects can also be widely styled using CSS. Large datasets can be easily bound to SVG objects using simple D3.js functions to generate rich text/graphic charts and diagrams. The data can be in various formats, most commonly JSON, comma-separated values (CSV) or geoJSON, but, if required, JavaScript functions can be written to read other data formats.

Selections

The central principle of D3.js design is to enable the programmer to first use a CSS-style selector to select a given set of Document Object Model (DOM) nodes, then use operators to manipulate them in a similar manner to jQuery. For example, by using D3.js, one may select all HTML <p>...</p> elements, and then change their text color, e.g. to lavender:

The selection can be based on tag (as in the above example), class, identifier, attribute, or place in the hierarchy. Once elements are selected, one can apply operations to them. This includes getting and setting attributes, display texts, and styles (as in the above example). Elements may also be added and removed. This process of modifying, creating and removing HTML elements can be made dependent on data, which is the basic concept of D3.js.

Transitions

By declaring a transition, values for attributes and styles can be smoothly interpolated over a certain time. The following code will make all HTML <p>...</p> elements on a page gradually change their text color to pink:

Data-binding

For more advanced uses, loaded data drives the creation of elements. D3.js loads a given dataset, then, for each of its elements, creates an SVG object with associated properties (shape, colors, values) and behaviors (transitions, events).

Generated SVG graphics are designed according to the provided data.

Appending nodes using data

Once a dataset is bound to a document, use of D3.js typically follows a pattern wherein an explicit .enter() function, an implicit "update," and an explicit .exit() function is invoked for each item in the bound dataset. Any methods chained after the .enter() command will be called for each item in the dataset not already represented by a DOM node in the selection (the previous selectAll()). Likewise, the implicit update function is called on all existing selected nodes for which there is a corresponding item in the dataset, and .exit() is called on all existing selected nodes that do not have an item in the dataset to bind to them. The D3.js documentation provides several examples of how this works.

API structure

D3.js API contains several hundred functions, and they can be grouped into following logical units:

Maths

Arrays

D3.js array operations are built to complement existing array support in JavaScript (mutator methods: sort, reverse, splice, shift and unshift; accessor methods: concat, join, slice, indexOf and lastIndexOf; iteration methods: filter, every, forEach, map, some, reduce and reduceRight). D3.js extends this functionality with:

Geometry

Color

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