[R] Suppressing error messages printed in xyplot() with panel function

[Adam Zeilinger]

Hello,

I am using the xyplot() function to create a series of scatterplot 
panels with lines of best fit.  To draw the lines of best fit for each 
panel, I am using a panel function.  Here's an example:

 > species <- as.character(c(rep(list("A", "B", "A"), 10), "B"))
 > year <- as.character(c(rep(list("2009", "2009", "2010"), 10), "2010"))
 > x <- rnorm(31, mean = 50, sd = 1)
 > y <- 3*x + 20
 > ex.data <- data.frame(cbind(species, year, x, y))
 > ex.data$x <- as.numeric(ex.data$x)
 > ex.data$y <- as.numeric(ex.data$y)
 > xyplot(y ~ x|species*year, data = ex.data,
+   panel = function(x, y) {
+   panel.xyplot(x, y, pch=16, col="black")
+   panel.abline(lm(y ~ x))})

With my data set, there are some panels with less than 2 data points.  
In these panels, an error message is printed in the panel, something 
like: "Error using packet 4 missing value where TRUE/FALSE needed."

In the panels with error messages, I want to keep the panels but 
suppress the error message, such that the panel is blank or has only one 
datum.  How do I do suppress the printing of the error message?

Thanks in advance for your help.
Adam

-- 

Adam Zeilinger
Conservation Biology Program
University of Minnesota