Linear Scale

A linear scale is an axis on which equal distances always represent equal differences in value, so the gap from 10 to 20 looks exactly the same size as the gap from 90 to 100.

This is the scale most charts use and the one most people assume by default. Because the relationship between value and position is constant, you can read a linear axis by simple proportion: a point twice as far from zero is twice the value. It is the natural choice whenever differences (how many more) are what matter to the reader.

How a linear scale works in a chart

On a linear scale the tick marks are placed at constant value increments — 0, 10, 20, 30 — and each step occupies the same amount of space. Mathematically, position is a straight-line function of value: doubling the value doubles the distance from the baseline. This is why bar charts, which compare lengths, almost always sit on a linear scale; the eye reads length as quantity only when the scale is linear.

A concrete example

Picture a vertical axis from 0 to 100 that is 200 pixels tall. On a linear scale each unit takes 2 pixels, so the value 50 sits at 100 pixels — exactly halfway up — and the value 25 sits at 50 pixels, a quarter of the way. The midpoint of the axis is always the midpoint of the value range. Contrast this with a log scale, where the midpoint would instead fall at 10, the geometric middle.

Related terms

A linear scale is one kind of scale applied to an axis; its opposite for spanning huge ranges is the log scale, where equal distance means equal ratio. See linear axes in everyday use in the line chart guide.