The input data for drawing curves with SAP Statistics is always a set of curve points. Each curve consists of a set of points, and each point has an x-value and a y-value.

Based on the curve points in the input, an SAP Statistics diagram may be one of two kinds:

- A diagram with a
*reference*axis

A reference axis (normally the x-axis) is an axis for which the curve values must be a fixed set. That is, if the x-axis is a reference axis, each curve in the diagram must have the exact same set of x-values.

Typically, the reference axis represents time. The curve values along this axis are specific dates.

The non-reference axis is the axis along which any values can occur.

The following figure contains an SAP Statistics graph. (With reference axis along the x-axis. The y-axis illustrates Common scaling mode.)

The values along the y-axis are stock prices for the given dates. You can compare stock prices in the two curves, where all prices (marked by gridlines) are given for the same set of dates.

The difference between reference and non-reference axes is important because of how SAP Statistics draws the scale for each axis:

- For a reference axis, the fixed set of values (or a selection of them) is used as scale values (that is, labels for the gridlines). Or, in special cases, (if the input data specifies explicitly that the axis measures times or dates), SAP Statistics can provide a special time line for scaling the axis (as in the first figure).

- For non-reference axes, since there is no common set of values, SAP Statistics generates the scale values itself. These scale values are not necessarily the same as the curve values along that axis. The difference is also important because some menu options apply only to reference axes, or only to non-reference axes.

The following figure shows a graph with no reference axes.