graphics - R package lattice won't plot if run using source()

Question

I started using the lattice graphic package but I stumbled into a problem. I hope somebody can help me out. I want to plot a histogram using the corresponding function.

Here is the file foo.r:

library("lattice")

data <- data.frame(c(1:2),c(2:3))
colnames(data) <- c("RT", "Type")

pdf("/tmp/baz.pdf")
histogram( ~ RT | factor(Type), data = data)
dev.off()

When I run this code using R --vanilla < foo.r it works all fine.

However, if I use a second file bar.r with

source("bar")

and run R --vanilla < bar.r the code produces an erroneous pdf file. Now I found out that source("bar", echo=TRUE) solves the problem. What is going on here? Is this a bug or am I missing something?

I'm using R version 2.13.1 (2011-07-08) with lattice_0.19-30

Answer 1

It is in the FAQ for R -- you need print() around the lattice function you call:

7.22 Why do lattice/trellis graphics not work?

The most likely reason is that you forgot to tell R to display the graph. Lattice functions such as xyplot() create a graph object, but do not display it (the same is true of ggplot2 graphics, and Trellis graphics in S-Plus). The print() method for the graph object produces the actual display. When you use these functions interactively at the command line, the result is automatically printed, but in source() or inside your own functions you will need an explicit print() statement.