[Rd] abline for intercept-only simple lm models (with and without offset)

[Tobias Verbeke]

The abline function can be used to draw the
regression line when one passes the lm object
as an argument.

However, if it's an intercept-only model,
it appears to use the intercept
as the slope of the abline:

mod <- lm(dist ~ 1, data = cars)
plot(dist ~ speed, data = cars)
abline(reg = mod) # nothing appears

This behaves as documented, but might catch
someone. Would it be an improvement
if this situation was detected so as to plot
the appropriate horizontal line, i.e.

abline(a = coef(mod), b = 0) ?

Would it also be an improvement if for
a model like

mod2 <- lm(dist ~ 1 + offset(speed), data = cars)

abline(reg = mod2) would be equivalent to

abline(a = coef(mod2), b = 1) ?

For models through the origin, the current
function works fine, but one might even
consider models through the origin and
having the independent variable in an offset()
to be fully fool-proof, i.e.

abline(a = 0, b = 1)

Kind regards,
Tobias