[R] unexpected behavior of trellis calls inside a user-defined function

[Jenny Bryan]

I am making a battery of levelplots and wireframes for several fitted  
models.  I wrote a function that takes the fitted model object as the  
sole argument and produces these plots.  Various strange behavior  
ensued, but I have identified one very concrete issue (illustrated  
below): when my figure-drawing function includes the addition of  
points/lines to trellis plots, some of the information (main title,  
placement of additional points) from my i-th function call is leaking  
over into the i+1-th call.  In the example below, I just get  
unexpected results.  In my actual application, it breaks the whole  
function and one of the error messages I've gotten is:

 > figFun(smoothFit1) ## no longer worked, once I got fancy with trellis
Error in depth(path) : no applicable method for "depth"

Smallest example I could construct to illustrate at least one of my  
problems:

predVals <-
   expand.grid(list(Sepal.Length = seq(from = min(iris$Sepal.Length),
                      to = max(iris$Sepal.Length), length = 50),
                    Petal.Length = seq(from =  min(iris$Petal.Length),
                      to = max(iris$Petal.Length), length = 50)))
irisFit <- lm(Sepal.Width ~ Sepal.Length * Petal.Length, data = iris)
predSurf <- data.frame(predVals, Sepal.Width = predict(irisFit,  
predVals))
trellis.device("X11",width = 8, height = 8)

levelplot(Sepal.Width ~ Sepal.Length * Petal.Length,
           predSurf, main = "Produced at command line, Take 1")

# put levelplot call inside a function
myFunction <- function(surf) {
   levelplot(Sepal.Width ~ Sepal.Length * Petal.Length,surf,
             main = "Produced by function, Take 1")
}
myFunction(predSurf)
# OK, get the expected figure result

## now .. what if we add points to the plot?
levelplot(Sepal.Width ~ Sepal.Length * Petal.Length,
           predSurf, main = "Produced at command line, Take 2")
trellis.focus("panel", 1, 1)     # address the correct part of the  
figure
lpoints(6,4,pch = 19, cex = 2)   # a point appears in correct location
# I get what I expect

## any crosstalk from adjacent command line invocations?  no
levelplot(Sepal.Width ~ Sepal.Length * Petal.Length,
           predSurf, main = "Produced at command line, Take 3")
trellis.focus("panel", 1, 1)     # address the correct part of the  
figure
lpoints(5,2,pch = 19, cex = 2)   # a point appears in correct location
# I get what I expect

# put all this inside a function
myFunction <- function(surf) {
   levelplot(Sepal.Width ~ Sepal.Length * Petal.Length,surf,
main = "Produced by function, Take 2")
trellis.focus("panel", 1, 1)     # address the correct part of the
lpoints(7,5,pch = 19, cex = 2)   # a point appears in correct location
}
myFunction(predSurf)
# the title still says "Produced at command line, Take 3"
# and points appear at (5,2) AND (7,5) ... why?