Scales are functions provided by D3.js that map a set of values to another set of values. The input set of values is referred to as the domain, and the output is the range. The basic reason for the existence of scales is to prevent us from coding loops, and doing a lot of math to make these conversions happen. This is a very useful thing.

There are three general categories of scales: quantitative, ordinal, and time-scale. Within each category of scale, D3.js provides a number of concrete implementations that exist for accomplishing a specific type of mapping data useful for data visualization.

Covering examples of every type of scale would consume more space than is available in this book, and at the same time become tedious to read. We will examine several common scales that are used—kind of the 80/20 rule, where the few we cover here will be used most of the time you use scales.