NULL. This function is used for its side effect: adding
a line to the plot.

Arguments

mod

a model, such as produced by lm,
that responds to the coef function
by returning a 2-element vector, whose elements are
interpreted respectively as the
intercept and slope of a regresison line.

col

color for points and lines; the default is the second entry
in the current car palette (see carPalette
and par).

In contrast to abline, this function plots only over
the range of the observed x-values. The x-values are extracted from
mod as the second column of the model matrix.