lattice xscale.components: different ticks on top/bottom axis

On Fri, Mar 11, 2011 at 12:28 AM,
<Boris.Vasiliev at forces.gc.ca> wrote:

Good afternoon,

I am trying to create a plot where the bottom and top axes have the
same scale but different tick marks. ?I tried user-defined
xscale.component function but it does not produce desired results.
Can anybody suggest where my use of xscale.component
function is incorrect?

For example, the code below tries to create a plot where horizontal
axes limits are c(0,10), top axis has ticks at odd integers, and
bottom axis has ticks at even integers.

library(lattice)

df <- data.frame(x=1:10,y=1:10)

xscale.components.A <- function(...,user.value=NULL) {
?# get default axes definition list; print user.value
?ans <- xscale.components.default(...)
?print(user.value)

?# start with the same definition of bottom and top axes
?ans$top <- ans$bottom

?# - bottom labels
?ans$bottom$labels$at <- seq(0,10,by=2)
?ans$bottom$labels$labels <- paste("B",seq(0,10,by=2),sep="-")

?# - top labels
?ans$top$labels$at <- seq(1,9,by=2)
?ans$top$labels$labels <- paste("T",seq(1,9,by=2),sep="-")

?# return axes definition list
?return(ans)
}

oltc <- xyplot(y~x,data=df,

scales=list(x=list(limits=c(0,10),at=0:10,alternating=3)),
? ? ? ? ? ? ? xscale.components=xscale.components.A,
? ? ? ? ? ? ? user.value=1)
print(oltc)

The code generates a figure with incorrectly placed bottom and top
labels. ?Bottom labels "B-0", "B-2", ... are at 0, 1, ... and top
labels "T-1", "T-3", ... are at 0, 1, ... ?When axis-function runs out
of labels, it replaces labels with NA.

It appears that lattice uses top$ticks$at to place labels and
top$labels$labels for labels. ?Is there a way to override this
behaviour (other than to expand the "labels$labels" vector to be as
long as "ticks$at" vector and set necessary elements to "")?

Well, <top|bottom>$ticks$at is used to place the ticks, and
$labels$at is used to place the labels. They should typically
be the same, but you have changed one and not the other.
Everything seems to work if you set $ticks$at to the same
values as $labels$at:


? ? ## ?- bottom labels
+ ? ans$bottom$ticks$at <- seq(0,10,by=2)
? ? ans$bottom$labels$at <- seq(0,10,by=2)
? ? ans$bottom$labels$labels <- paste("B",seq(0,10,by=2),sep="-")

? ? ## ?- top labels
+ ? ans$top$ticks$at <- seq(1,9,by=2)
? ? ans$top$labels$at <- seq(1,9,by=2)
? ? ans$top$labels$labels <- paste("T",seq(1,9,by=2),sep="-")

Also, can user-parameter be passed into xscale.components()
function? (For example, locations and labels of ticks on the top
axis). ?In the ?code above, print(user.value) returns NULL even
though in the xyplot() call user.value is 1.

No. Unrecognized arguments are passed to the panel function
only, not to any other function. However, you can always
define an inline
function:

oltc <- xyplot(y~x,data=df,
? ? ? ? ? ? ? ?scales=list(x=list(limits=c(0,10), at = 0:10,
? ? ? ? ? ? ? ? ? ? ? ? ? ?alternating=3)),
? ? ? ? ? ? ? ?xscale.components = function(...)
? ? ? ? ? ? ? ? ? ? ? ? ? ?xscale.components.A(..., user.value=1))

Hope that helps (and sorry for the late reply).

-Deepayan

Deepyan,

Thank you very much for your reply. ?It makes things a bit clearer.

It other words in the list prepared by xscale.components(), vectors
<top|bottom>$ticks$at and <top|bottom>$labels$at must be the same.
If only every second tick is to be labelled then every second label
should be set explicitly to empty strings: