>>>>> "BXC" == BXC (Bendix Carstensen) <[EMAIL PROTECTED]>
>>>>> on Thu, 15 Apr 2004 09:54:49 +0200 writes:
BXC> Is there a function that does the same as pretty but on
BXC> a log-scale? Suppose you have
BXC> x <- exp( runif( 100, 0, 6 ) )
BXC> (which will between 1 and 403), then I would like to
BXC> have a result like:
BXC> log.pretty( x )
BXC> [1] 1 5 10 50 100 500
axTicks() can be used for this and will use the same algorithm
that R uses internally; it's not as flexible as pretty() is,
but should fulfill your needs.
As I (author of the function) just see, the documentation of
axTicks is insufficient:
?axTicks points to ?par ("xaxp") which points
back to ?axTicks without explaining the crucial meaning of axp[3]
for the log case:
axp[3] values
------ ---------------
1 1 * 10^j
2 (1,5) * 10^j
3 (1,2,5) * 10^j
Also, contrary to help(axTicks), the result of axTicks() does
not depend on the graphics state at all --- *when* all arguments
are specified (differently from the default NULL).
Here is an example "script":
## just to "repeat your example" :
> set.seed(1); x <- exp(runif(100,0,6)); rx <- range(x); summary(x)
Min. 1st Qu. Median Mean 3rd Qu. Max.
1.084 6.950 18.680 62.230 99.800 384.300
> axTicks(1, axp=c(rx, 1), usr=c(1,3), log=TRUE)
[1] 10 100 1000
> axTicks(1, axp=c(rx, 2), usr=c(1,3), log=TRUE)
[1] 10 50 100 500 1000
> axTicks(1, axp=c(rx, 3), usr=c(1,3), log=TRUE)
[1] 10 20 50 100 200 500 1000
> axTicks(1, axp=c(rx, 3), usr=c(0,3), log=TRUE)
[1] 1 2 5 10 20 50 100 200 500 1000
--- note that "usr" is in the log-transformed scale.
Probably the whole thing is most easily understood by looking at
the source (*including* the comments you usually won't see!)
of which the relevant part is
........
if(log && axp[3] > 0) { ## special log-scale axp[]
if(!any((iC <- as.integer(axp[3])) == 1:3))
stop("invalid positive axp[3]")
if(is.null(usr)) usr <- par("usr")[if(is.x) 1:2 else 3:4]
else if(!is.numeric(usr) || length(usr) != 2) stop("invalid `usr'")
ii <- round(log10(axp[1:2]))
x10 <- 10^((ii[1] - (iC >= 2)):ii[2])
r <- switch(iC, ## axp[3]
x10, ## 1
c(outer(c(1, 5), x10))[-1],## 2
c(outer(c(1,2,5), x10))[-1])## 3
r[usr[1] <= log10(r) & log10(r) <= usr[2]]
} else { # linear
........
Hoping that helps,
Martin Maechler
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