problem with plotting .pngs with the rasterVis package

hello all,
I got a problem with the rasterVis package.
I tried to do some plots in a loop and write every plot with Cairo to
disc. But the picture in the file on the disc is totaly black or a
white.

Do somebody know how to make png/pdf levelplots in a loop?

here is my test-script
# problem with RasterVis plots in loops
#
###############################################################################

library(raster)
library(Cairo)
library(rasterVis)

# create RasterLayer / Stack
r <- raster(nrows=10, ncols=10)
r <- setValues(r, 1:ncell(r))
s <- stack(r,r*2)


# single plot is done
CairoPNG(width = 400, height = 400, filename="test.png")
   levelplot(s)
dev.off()

# plot in a loop: no plots are done!
for(i in 1:3){
   print(paste("Cairo loop",i))
   levelplot(s*i)
}

# plot in a loop with cairo: plots are black
for(i in 1:3){
   print(paste("Cairo loop",i))
   CairoPNG(width = 400, height = 400, filename="test.png")
       levelplot(s*i)
   dev.off()
}

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