9.4 A Module to Add Lines to a Graph

In creating statistical graphs, it is often useful to overlay reference lines. The simplest reference line is a horizontal line. For example, you might want to add a reference line at zero on a residual plot in order to separate positive residuals from negative. It is often useful to display the identity line. In regression diagnostics (see Chapter 12), it is useful to display lines that have statistical significance, such as lines that indicate large residuals or high-leverage values for observations.

Reference lines are used so frequently that it is worthwhile to construct a SAS/IML subroutine to draw a reference line. A convenient syntax for a module that overlays the line defined by the equation y = a + bx might be as follows: