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axTicks: Compute Axis Tickmark Locations

axTicks

R Documentation

Compute Axis Tickmark Locations

Description

Compute pretty tickmark locations, the same way as R does internally. This is only non-trivial when log coordinates are active. By default, gives the at values which axis(side) would use.

Usage

axTicks(side, axp = NULL, usr = NULL, log = NULL, nintLog = NULL)

Arguments

`side`	integer in 1:4, as for `axis`.
`axp`	numeric vector of length three, defaulting to `par("xaxp")` or `par("yaxp")` depending on the `side` argument (`par("xaxp")` if `side` is 1 or 3, `par("yaxp")` if side is 2 or 4).
`usr`	numeric vector of length two giving user coordinate limits, defaulting to the relevant portion of `par("usr")` (`par("usr")[1:2]` or `par("usr")[3:4]` for `side` in (1,3) or (2,4) respectively).
`log`	logical indicating if log coordinates are active; defaults to `par("xlog")` or `par("ylog")` depending on `side`.
`nintLog`	(only used when `log` is true): approximate (lower bound for the) number of tick intervals; defaults to `par("lab")[j]` where `j` is 1 or 2 depending on `side`. Set this to `Inf` if you want the same behavior as in earlier R versions (than 2.14.x).

Details

The axp, usr, and log arguments must be consistent as their default values (the par(..) results) are. If you specify all three (as non-NULL), the graphics environment is not used at all. Note that the meaning of axp differs significantly when log is TRUE; see the documentation on par(xaxp = .).

axTicks() may be seen as an R implementation of the C function CreateAtVector() in ‘..../src/main/plot.c’ which is called by axis(side, *) when no argument at is specified or directly by axisTicks() (in package grDevices).
The delicate case, log = TRUE, now makes use of axisTicks unless nintLog = Inf which exists for back compatibility.

Value

numeric vector of coordinate values at which axis tickmarks can be drawn. By default, when only the first argument is specified, these values should be identical to those that axis(side) would use or has used. Note that the values are decreasing when usr is (“reverse axis” case).

Examples

 plot(1:7, 10*21:27)
 axTicks(1)
 axTicks(2)
 stopifnot(identical(axTicks(1), axTicks(3)),
           identical(axTicks(2), axTicks(4)))

## Show how axTicks() and axis() correspond :
op <- par(mfrow = c(3, 1))
for(x in 9999 * c(1, 2, 8)) {
    plot(x, 9, log = "x")
    cat(formatC(par("xaxp"), width = 5),";", T <- axTicks(1),"\n")
    rug(T, col =  adjustcolor("red", 0.5), lwd = 4)
}
par(op)

x <- 9.9*10^(-3:10)
plot(x, 1:14, log = "x")
axTicks(1) # now length 5, in R <= 2.13.x gave the following
axTicks(1, nintLog = Inf) # rather too many

## An example using axTicks() without reference to an existing plot
## (copying R's internal procedures for setting axis ranges etc.),
## You do need to supply _all_ of axp, usr, log, nintLog
## standard logarithmic y axis labels
ylims <- c(0.2, 88)
get_axp <- function(x) 10^c(ceiling(x[1]), floor(x[2]))
## mimic par("yaxs") == "i"
usr.i <- log10(ylims)
(aT.i <- axTicks(side = 2, usr = usr.i,
                 axp = c(get_axp(usr.i), n = 3), log = TRUE, nintLog = 5))
## mimic (default) par("yaxs") == "r"
usr.r <- extendrange(r = log10(ylims), f = 0.04)
(aT.r <- axTicks(side = 2, usr = usr.r,
                 axp = c(get_axp(usr.r), 3), log = TRUE, nintLog = 5))

## Prove that we got it right :
plot(0:1, ylims, log = "y", yaxs = "i")
stopifnot(all.equal(aT.i, axTicks(side = 2)))

plot(0:1, ylims, log = "y", yaxs = "r")
stopifnot(all.equal(aT.r, axTicks(side = 2)))