X-Axis

The x-axis is the horizontal reference line of a chart, running left to right, and it most often holds the categories or the time periods being plotted.

By long-standing convention the x-axis represents the "independent" variable — the thing you organise the data by, such as months, product names, or ages — while the vertical axis carries the value that responds to it. Reading left to right along the x-axis is how you move through the categories or forward through time, which is why trends in a time series appear to travel rightward.

How the x-axis works

The x-axis can be one of three kinds. A categorical x-axis lists discrete labels at even spacing, like the names of teams or regions. A time x-axis spreads dates along its length so that gaps in time show as gaps in space. A numeric x-axis carries a continuous range of numbers, as in a scatter plot where both axes are measurements. The labels and tick marks running beneath the line tell you what each horizontal position means.

A concrete example

In a line chart of website visits over a year, the x-axis runs across the bottom with the twelve months spaced evenly from January on the left to December on the right. To find the value for a given month, you locate that month on the x-axis, move straight up to the line, and then read across to the vertical axis. The x-axis answers "when?" or "which one?"; the vertical axis answers "how much?".

Related terms

The x-axis is one half of a chart's pair of axes; its partner is the y-axis. How positions map to values along it is set by the scale. A horizontal time axis is what makes a line chart read as a left-to-right story.