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

[Boris.Vasiliev at forces.gc.ca]

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 "")?

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.

Sincerely,
Boris Vasiliev.