Axes are lines that provide references for measurement or comparison.

The primary axis (or Y-axis) refers to measures of quantitative data, such as sales figures or quantities. Visualizations can have more than one primary axis.

The category axis (X-axis or ordinal axis) plots qualitative data, such as products or regions. It runs horizontally, except in bar charts.

The z-axis is the vertical axis in a 3-D chart.

Major grid lines extend from the tick marks on an axis and run behind the data markers.