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

Martin Maechler maechler at stat.math.ethz.ch
Mon Dec 11 14:17:48 CET 2006

```Yes, I think  all your propositions would be improvements.
OTOH, I don't think the improvements warrant a big increase in
code (complexity), nor do I think the improvements are crucial
for R's integrity.

So...
If you (our someone else) provides a patch {against
R-devel, as always} which keeps the code simple,
I'd strongly consider adding that to R.

Regards,
Martin Maechler

>>>>> "Tobias" == Tobias Verbeke <tobias.verbeke at telenet.be>
>>>>>     on Sat, 09 Dec 2006 22:42:38 +0100 writes:

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

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

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

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

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

Tobias> Would it also be an improvement if for a model like

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

Tobias> abline(reg = mod2) would be equivalent to

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

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

Tobias> abline(a = 0, b = 1)

Tobias> Kind regards, Tobias

Tobias> ______________________________________________
Tobias> R-devel at r-project.org mailing list
Tobias> https://stat.ethz.ch/mailman/listinfo/r-devel

```