r - Lattice plots in for loop - empty images created

Question

I want to produce several lattice plots in a for loop, but it does create empty images!!!

for (f in unique(df$month)) {
    plot.new()
    bwplot(x ~ country|type, data = df[df$month == f,], panel=function(...) {
        panel.abline(h=0, col="green")
        panel.bwplot(...)
    })
    savePlot(paste0("file_", f), "png")
}

When I run the inner of the for loop "by hand", it works, but in loop it stops working. Why?

Here is the code to generate the data:

set.seed(123)
n <- 300
country <- sample(c("Europe", "Africa", "Asia", "Australia"), n, replace = TRUE)
type <- sample(c("city", "river", "village"), n, replace = TRUE)
month <- sample(c("may", "june", "july"), n, replace = TRUE)
x <- rnorm(n)
df <- data.frame(x, country, type, month)

Answer 1

The first part is an FAQ (in R docs, and on SO): you must print(mylatticeplot), when not interactive.

In addition, your approach does not work in RStudio, for example.

Error in savePlot(paste0("file_", f), "png") : 
  can only copy from 'windows' devices

The recommended way works better, and is less work:

png("file_%03d.png")
for (f in unique(df$month)) {
  p = bwplot(x ~ country|type, data = df[df$month == f,], panel=function(...) {
    panel.abline(h=0, col="green")
    panel.bwplot(...)
  })
  print(p)
}
dev.off()