The LAYOUT OVERLAYEQUATED Statement

Several SAS procedures create plots where the X and Y axes are scaled in the same units. Here are some samples of such plots taken from the Examples section of the procedure documentation.

Sample Plot from PROC PRINQUAL

Sample Plot from PROC LOGISTIC

Sample Plot from PROC PLS

Whenever the same units of measure are used on both axes, it is desirable that the distance displayed between the same data interval be the same on both axes. To achieve this effect, you must use an OVERLAYEQUATED layout.

For specifying plot statements, the OVERLAYEQUATED layout is similar to the OVERLAY layout: you can specify one or more 2D plot statements within the layout block. However, OVERLAYEQUATED imposes a restriction on the plot axes and differs from OVERLAY in several ways. With OVERLAYEQUATED,

Both X and Y axes are always numeric (TYPE=LINEAR). Thus, plot types that have discrete or binned axes (BOXPLOT, BOXPLOTPARM, BARCHARTPARM, HISTOGRAM, and HISTOGRAMPARM ) cannot be used within this layout.
For equal data intervals on both axes, the display distance is the same. For example, an interval of 2 units on the X axis maps to the same display distance as an interval of 2 units on the Y axis.
The slope of a line in the display is the same as the slope in the data. In other words, a 45° slope in data will be represented by a 45° slope in the display. The EQUATETYPE= option offers different ways of presenting the data ranges while preserving the 45° display slope (see Types of Equated Axes).

The following figure illustrates how a series plot might be displayed when it is specified within an OVERLAYEQUATED layout rather than an OVERLAY layout:

OVERLAYEQUATED layout contrasted to OVERLAY layout