R help - Sep 2010 - Odd graphics output problem

Hi all,

     I'm having trouble saving graphics output from within a loop, and I
can't figure out a solution.  I'd like to produce and save lots of
individual plots for inspection, so I set up the following script:

library( lattice )
wd = "~/Documents/PPM/"
ppm = read.table( paste( wd, "ppm_summary.txt", sep = "" ),
sep = "\t",
header = TRUE )
ppm$Trial = as.factor( ppm$Trial )

for ( ID in levels( ppm$Patient ) ){
    patient_data = ppm[ ppm$Patient == ID, ]
    dir.create( paste( wd, ID, sep = "" ) )
    for ( trial in ( levels( ppm[ ppm$Patient == ID, ]$Trial ) ) ){
        trial_data = patient_data[ patient_data$Trial == trial, ]
        trial_str = sprintf( "%03d", as.integer( trial ) )
        path = paste( wd, ID, "/", trial_str, ".png", sep =
"" )
        cat( path, "\n" )
        png( path )
        xyplot( PupilArea ~ Offset, data = trial_data )
        dev.off()
    }
}

     Everything runs, the cat() command displays output to the console, but
no files are created.  However, if I interrupt the script with control-C and
execute the png(), xyplot(), and dev.off() commands manually, the
appropriate plot image files are written based on the correct data.  That
is, everything from data subsetting to plotting works fine except when the
loop is actually running.  I've tried swapping png() for bitmap(),
Sys.sleep()ing R at various stages, and some other things with no success.
What am I missing?  Thanks in advance,

Dave

-- 
Post-doctoral Fellow
Neurology Department
University of Iowa Hospitals and Clinics
davideugenewarren@gmail.com

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See the FAQ 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*<http://cran.r-project.org/package=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.

On Thu, Sep 16, 2010 at 10:34 AM, David Warren
<davideugenewarren@gmail.com>wrote:
> Hi all,
>
>     I'm having trouble saving graphics output from within a loop, and I
> can't figure out a solution.  I'd like to produce and save lots of
> individual plots for inspection, so I set up the following script:
>
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