bug in qplot (library ggplot2)

Hello,

this following code give a nice png:

/library(ggplot2)
i<- 1
png(file=paste('test ',i,'.png',sep=''))
qplot(carat, data=diamonds,
fill=color,geom='histogram')+scale_y_continuous(i)
dev.off()
/
I would like to get more files, but the following code doesn't make any
file:

/library(ggplot2)
for (i in 1:2) {
      png(file=paste('test ',i,'.png',sep=''))
      qplot(carat, data=diamonds,
fill=color,geom='histogram')+scale_y_continuous(i)
      dev.off()
}
/
Have you got any idea?
Have I made something false?

Thanks,

Pierre-Olivier

R version 2.12.1 (2010-12-16)
Platform: i386-redhat-linux-gnu (32-bit)


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