R devel - Dec 2006 - abline for intercept-only simple lm models (with and without offset)

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

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> ______________________________________________
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    Tobias> https://stat.ethz.ch/mailman/listinfo/r-devel