# [R] abline() or predict.lm() when log="x"

Richard Chandler rchandler at forwild.umass.edu
Fri Jan 20 23:30:18 CET 2006

```Thanks for the reply though I don't think your suggestion worked. I
have found a way to get the correct line though it is not
convenient.  Here is a better example:

x <- 1:100
y <- 1:100
plot(y ~ x, log="x")

#The only way I can get the correct line is to drop the log():
abline(lm(y ~ x), untf=T, lwd=2) #or
lines(x, predict(lm(y ~ x)), col=2)

#Neither of these work
abline(lm(y ~ log10(x))) #or
abline(lm(y ~ log10(x)), untf=T)

What I really would like to do is plot fitted lines and 95%
confidence intervals using predict.lm, as in shown in the example,
but when the predictor is log transformed and log="x". I can't figure
out how to do this without removing the log() from the response part
of the formula and this isn't helpful because I'm generally trying to
give predict() a fitted object rather than a lm() formula. I still
think I'm probably missing something simple but are there any other
suggestions? Thanks.

Richard

Quoting Peter Dalgaard <p.dalgaard at biostat.ku.dk>:

> Richard Chandler <rchandler at forwild.umass.edu> writes:
>
> > Hello,
> >
> > I'm trying to plot a fitted lm() line on a plot when the one
> > explanatory variable is log transformed and log="x". I get
> different
> > lines using abline and predict.lm().
> >
> > #Example
> > x <- 1:100
> > y <- rnorm(100)
> > plot(y ~ x, log="x")
> > abline(lm(y ~ log(x)))
> > lines(x, predict(lm(y ~ log(x))), lwd=2)
> >
> > I'm sure I'm missing something but could someone tell me which
> line is
> > correct? Thanks.
>
> Base 10 is what you're missing.
>
> The latter form is agnostic with respect to base, the former is
> not
> (since the fitted values are the same, but regression coefficients
> differ). So you need to know to use abline(lm(y ~ log10(x))).
>
> You don't really notice which kind of log is being used until you
> look
> at par(usr) for a plot with logged axes.
>
> --
>    O__  ---- Peter Dalgaard             Øster Farimagsgade 5,
> Entr.B
>   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
>  (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45)
> 35327918
> ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45)
> 35327907
>

--
Richard Chandler, M.S. candidate
Department of Natural Resources Conservation
UMass Amherst
(413)545-1237
----- End forwarded message -----

--
Richard Chandler, M.S. candidate
Department of Natural Resources Conservation
UMass Amherst
(413)545-1237

```