That's easy too. Firstly change the print statement so it would print the time (in epoch seconds) and the rate separated by spaces. Write that output to a file, and then graph that file with gnuplot.

I actually have a perl program that draws a graph by calling gnuplot, here are a few parts of its code (I did it as a work so I don't want to publish all of it). The part not shown here opens a tempfile and writes into it the numbers in the simple format mentioned above. I also print some overall statistics to stdout. The following sub graphs the data then (it might need some adaptation for your purposes).

use File::Temp ();

sub showgraph {
        my($cmdfile, $cmdfilename);
        my $cmds = qq[set key off\nplot "] . quote($outname) . qq[" wi
+th lines\n! echo "Press return to continue"
; read\n];
        ($cmdfile, $cmdfilename) = File::Temp::tempfile undef, "UNLINK
+", 1;
        $out or die "error creating temporary commands file";
        print "Writing gnuplot command to temporary file $cmdfilename\
+n";
        print $cmdfile $cmds or
                die "error writing command file: $!";
        flush $cmdfile or die
                "error flushing comand file";
        print "Launching gnuplot\n";
        system "gnuplot", $cmdfilename;
        close $cmdfile;
}
[download]

Update 2008-01-29: see Plot a spiral with gnuplot.