Lattice plot with both a second x- and a second y-axis?

Question

I would like to add a 2nd y-axis (right) and a 2nd x-axis (top) to the following (lattice) levelplot. These axes should only indicate certain rows and columns (no labels) and thus mimick base-graphics' rug function. How can this be done?

library(lattice)
library(latticeExtra)

## Generate a correlation matrix
d <- 50
L <- diag(1:d)
set.seed(271)
L[lower.tri(L)] <- runif(choose(d,2))
Sigma <- L %*% t(L)
P <- cor(Sigma)

## Panel function
my_panel <- function(...) {
    panel.levelplot(...)
    panel.abline(h = (1:4)*10, v = (1:4)*10, lty = 2)
    panel.axis(side = "top", at = (1:50)-0.5, draw.labels = FALSE) # maybe a panel axis could do it? why not centered?
}

## Plot
obj1 <- levelplot(P, xlab = "Column", ylab = "Row",
                  col.regions = grey(c(seq(1, 0, length.out = 600))),
                  panel = my_panel)
obj2 <- xyplot(NA~NA, ylim = c(0, 50),
               scales = list(x = list(at = (1:50)-0.5, labels = rep("", 50)),
                             y = list(at = (1:50)-0.5, labels = rep("", 50))))
doubleYScale(obj1, obj2, use.style = FALSE) # idea based on latticeExtra; only gives a 2nd y-axis, though

Answer 1

You were onto a good idea with panel.rug(), but were stymied by lattice's default clipping of its plotting to the panel's interior. To get around that, you can turn off clipping via the par.settings= argument. If you want to suppress the plotting of default axis tick marks on the right and top panel borders, you can do so using the tck= argument, as shown below.

my_panel <- function(...) {
    panel.levelplot(...)
    panel.abline(h = (1:4)*10, v = (1:4)*10, lty = 2)
    ## Have panel.rug print tick marks starting at 1 npc (edge of panel)
    ## and extending to 1.02 npc (slightly outside of panel). (See ?unit)
    panel.rug(x = (1:51)-0.5, y = (1:51)-0.5, 
              start = 1, end = 1.02,
              col="black")
}

levelplot(P, xlab = "Column", ylab = "Row",
          col.regions = grey(c(seq(1, 0, length.out = 600))),
          ## Suppress default scales on right and top sides, by setting their
          ## tick lengths to zero 
          scales = list(tck=c(1,0)),
          ## Turn off clipping, so that panel.rug can plot outside of the panel
          par.settings = list(clip = list(panel = "off")),          
          panel = my_panel)