# [R] plotmath question

From: Berton Gunter <gunter.berton_at_gene.com>
Date: Sat 19 Mar 2005 - 08:41:22 EST

R listers:

I have been foiled by plotmath!

(in R 2.01,Windows 2000)

The task: Plot a normal density and label the ticks as mu - 3 sigma, mu - 2 sigma, ...., mu + 3 sigma, where the mu's and sigmas appear as Greek symbols, of course.

The following code does this:

x<-seq(-3,to=3,by=.01)
y<-dnorm(x)
plot(x,y,type='h',col='lightblue',axes=FALSE) lines(x,y,col='darkblue')
axis(2)
for(i in seq(-3,to=3))

```	axis(1,at=i, lab=switch(sign(i)+2,
eval(substitute(expression(mu-j*sigma),list(j=-i))),
expression(mu),
eval(substitute(expression(mu+j*sigma),list(j=i)))))
```
box()

However, I think the code in the for loop is ugly and probably means that I'm doing it wrong. In particular:

1. Is there a neat way to use one axis() call and a vector (of expressions?) for the lab=argument?
2. The plotmath Help state that expressions can be used for axis labels, so I would have expected the above to work without the eval()call -- but it does not. Would someone kindly explain to me why not -- i.e., what I have misunderstood. That is, to be clear, why does the following not work:

for(i in seq(-3,to=3))

```	axis(1,at=i, lab=switch(sign(i)+2,
substitute(expression(mu-j*sigma),list(j=i)),
expression(mu),
substitute(expression(mu+j*sigma),list(j=i))))

```

Any further ideas,insights, or pointers to reference materials would also be appreciated. Many thanks.

• Bert Gunter Genentech Non-Clinical Statistics South San Francisco, CA

"The business of the statistician is to catalyze the scientific learning process." - George E. P. Box

R-help@stat.math.ethz.ch mailing list