Book Image

Learn Chart.js

By : Helder da Rocha
Book Image

Learn Chart.js

By: Helder da Rocha

Overview of this book

Chart.js is a free, open-source data visualization library, maintained by an active community of developers in GitHub, where it rates as the second most popular data visualization library. If you want to quickly create responsive Web-based data visualizations for the Web, Chart.js is a great choice. This book guides the reader through dozens of practical examples, complete with code you can run and modify as you wish. It is a practical hands-on introduction to Chart.js. If you have basic knowledge of HTML, CSS and JavaScript you can learn to create beautiful interactive Web Canvas-based visualizations for your data using Chart.js. This book will help you set up Chart.js in a Web page and show how to create each one of the eight Chart.js chart types. You will also learn how to configure most properties that override Chart’s default styles and behaviors. Practical applications of Chart.js are exemplified using real data files obtained from public data portals. You will learn how to load, parse, filter and select the data you wish to display from those files. You will also learn how to create visualizations that reveal patterns in the data. This book is based on Chart.js version 2.7.3 and ES2015 JavaScript. By the end of the book, you will be able to create beautiful, efficient and interactive data visualizations for the Web using Chart.js.
Table of Contents (14 chapters)
Title Page
Copyright and Credits
About Packt


There is no native support in Chart.js for gradients, but they are fairly easy to generate with Canvas. The problem is that a gradient has an absolute position in a Canvas object, while your chart may be responsive. If the chart is resized, the gradient has to be recalculated and the chart updated.


One way to deal with this is to call a gradient function as soon as the chart is created and every time the window is resized, feeding the Canvas gradient function with the dimensions of the area where the gradient will be applied. We can do this with a callback and the Chart.js update() function.

A gradient in Canvas is created with the following function:

canvasContext.createLinearGradient(x0, y0, x1, y1);

The gradient contains the equation of a perpendicular line. To create a linear gradient that varies along the axis, we need to draw the line from the bottom of the chart to the top. That means that x0 = x1 = 0, y1 is the bottom of the chart, and y0 is the top. If we write a function...