Understanding the axes of a chart
Most charts have horizontal and vertical axes, called the x-axis and y-axis. These axes usually display the following types of values:
Category series
A set of values that lie on the x-axis. Category series can be of any data type.
Value series
A set of values that lie on the y-axis. A value series can be of the numeric, date, or date-and-time data type, depending on the chart type.
Generally, a chart displays a single category on the x-axis, and one or more value series on the y-axes. An axis displays additional value series or group values for a category series parallel to the x-axis. The series that is parallel to the x-axis often appears in depth or three dimensions. Certain charts, such as pie and meter charts, do not have any axes.