R help - Jul 2014 - Weird but interesting behavior of ggplot2 in a loop

Hello and sorry I am not providing an example. However, I hope you'll be
able to answer my question.

I have a "for" loop. Inside this loop I build 4 plots using ggplot2.
See 2
blocks of code below - I have in total 4 of those:
As I step through the loop manually and run all the code inside the loop,
everything works perfectly and R generates (among other things) 4 .png
files in my working directory (each size 7 KB).

However, when I run the loop (even when the loop length is 1) the generated
.png files are empty (and size 1 KB). I've never seen anything like this.
My loop is correct - because everything else generated in the loop except
for the images is there. And the image files with right names are also
there, they are just empty.

Any ideas why this might be happening? I did it first in RStudio and then
just in R Gui.
Thank you very much!



  png(forname1,width=700,height=450)
  ggplot(result.opt, aes(x=prices, y=demand))+
    geom_line(colour='darkblue',size=1.2)+

scale_x_continuous(breaks=seq(input[1,1],input[nrow(input),1],by=step*6))+
    scale_y_continuous(breaks=seq(0,1,by=0.1),limits=c(0,1))+
    xlab("Price") + ylab("Purchase Intent")+
    labs(title=forname2)+
    theme(plot.title = element_text(size=16))+
    theme(axis.title.x=element_text(size=12))+
    theme(axis.title.y=element_text(size=12))
  dev.off()

  png(forname2,width=700,height=450)
  ggplot(result.opt, aes(x=prices, y=revenue))+
    geom_line(colour='blue',size=1.2)+

scale_x_continuous(breaks=seq(input[1,1],input[nrow(input),1],by=step*6))+
    # scale_y_continuous(breaks=seq(0,1,by=0.1),limits=c(0,1))+
    xlab("Price") + ylab("Revenue per Person")+
    labs(title=forname2)+
    theme(plot.title = element_text(size=16))+
    theme(axis.title.x=element_text(size=12))+
    theme(axis.title.y=element_text(size=12))
  dev.off()


-- 
Dimitri Liakhovitski

	[[alternative HTML version deleted]]