R help - May 2009 - quick square root axes

Dear R users,
while I enjoy the built-in log argument to the plot() function, I wished it
would be as easy to create more general custom transformed axes such as
sqrt(), logit, etc...

for example, instead of
 plot(x=exp(rnorm(10)), y=(1:10)^4, log = "xy"), sth. along the lines
of
 plot(x=exp(rnorm(10)), y=(1:10)^4, trans = list(x = log, y = sqrt))
to encode the desired transfomation.

This involves just transforming the xy values and creating nice tick marks
at the appropriate positions.
Before trying to write my own function, I wanted to see if that
functionality already exists in another package ?

Thanks!

Markus
.

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On 5/5/2009 1:05 PM, Markus Loecher wrote:
> Dear R users,
> while I enjoy the built-in log argument to the plot() function, I wished it
> would be as easy to create more general custom transformed axes such as
> sqrt(), logit, etc...
> 
> for example, instead of
>  plot(x=exp(rnorm(10)), y=(1:10)^4, log = "xy"), sth. along the
lines of
>  plot(x=exp(rnorm(10)), y=(1:10)^4, trans = list(x = log, y = sqrt))
> to encode the desired transfomation.
> 
> This involves just transforming the xy values and creating nice tick marks
> at the appropriate positions.
> Before trying to write my own function, I wanted to see if that
> functionality already exists in another package ?
I don't know of such a thing, but it may well exist.

If you do write your own, the hardest part will be picking the nice tick 
marks.  They should be approximately evenly spaced, but at nice round 
values of the original variable:  that's hard to do in general.  R has 
the pretty() function for the linear scale, and doesn't do too badly on 
log axes, but you'll need to work out your own rules for the sqrt or 
other scales.

Duncan Murdoch

Markus Loecher wrote:
> Dear R users,
> while I enjoy the built-in log argument to the plot() function, I wished it
> would be as easy to create more general custom transformed axes such as
> sqrt(), logit, etc...
>
> for example, instead of
>  plot(x=exp(rnorm(10)), y=(1:10)^4, log = "xy"), sth. along the
lines of
>  plot(x=exp(rnorm(10)), y=(1:10)^4, trans = list(x = log, y = sqrt))
> to encode the desired transfomation.
>
> This involves just transforming the xy values and creating nice tick marks
> at the appropriate positions.
> Before trying to write my own function, I wanted to see if that
> functionality already exists in another package ?
>
>   
Hi Markus,
The axis.mult function in plotrix does this for linear multipliers, but 
a more general transformation is a bit harder. I'll have a look at the 
log axis transformation and scratch my head a bit.

Jim

Here's a Q&D bit of code I wrote up a while back.  Dunno if it'll be
of
any use in setting up your tickmarks.

# x, n same meaning as in pretty()
# update: check for x neg or zero
# ... is for other args to pretty()
prettylog<- function(x,n=10,base=10,...) {
	x<-unlist(x)
	if (min(x)<=0) {
		print("Warning: nonpositive values removed")
		
		x<-x[c(x>0)]
		# just to avoid a single-value fed to pretty()
		if(length(x)==1) x<-c(1,x)
		}
	return((base^(pretty(log(x,base=base),n=n,...))))
	}