Y-Axis

The y-axis is the vertical reference line of a chart, running bottom to top, and it most often holds the numeric value being measured.

If the horizontal axis answers "which category?" or "when?", the y-axis answers "how much?". It is conventionally the "dependent" variable — the quantity that rises or falls in response to whatever the horizontal axis organises. Because height is read instantly, the y-axis is where the magnitude of your data lives: taller bars and higher points simply mean bigger numbers.

How the y-axis works

The y-axis is usually numeric. It runs from a minimum at the bottom to a maximum at the top, with tick marks at regular intervals and a title naming the quantity and its units (for example, "Sales (USD)"). The position of each tick is determined by the axis's scale. The single most important decision for a y-axis is where it begins: for bar charts it should start at zero, because the bar's height is meant to be read as a true proportion of the value.

A concrete example

In a column chart of monthly rainfall, the y-axis runs up the left side from 0 at the bottom to the wettest month at the top, labelled in millimetres. A bar that reaches the gridline at 80 represents 80 mm. To read any value, follow the top of a bar horizontally across to the y-axis and note the number. If that axis had started at 40 instead of 0, a month with 80 mm would look twice as tall as one with 60 mm — a visual exaggeration of a real but smaller difference.

Related terms

The y-axis is the vertical member of a chart's pair of axes, complementing the x-axis. Its range and spacing come from the scale, and the choice of starting point is discussed in the bar chart guide.