The previous posters gave excellent input. Allow me to expand and enhance. I recently put quite a bit of time into understanding gnuplot. Be warned that the options available to you in a graphics window are different than those that are available when you print to a file. This, and the non-orthogonal arrangement of options, caused my quite a lot of consternation. I eventually decided gnuplot did not suit my data set so I ended up switching back to GD and it's offspring. Now I'm much happier.
I case you wind up going that route, here is a perl script to launch gnuplot and feed it a list of commands. The commands generate a png file.
#!/usr/bin/perl
#
# ptoplot.pl
#
use strict;
use warnings;
my $output_file = "graphics/plot1";
my $cmd = '| gnuplot - 2>&1';
open PLOT, "$cmd" or die;
while (<DATA>) {
print PLOT "$_";
}
qx("sync");
__DATA__
set terminal png medium color picsize 800 600
set output "graphics/plot1"
set title 'Splot of file plot.dat'
set style data dots
# set hidden3d
# set data palette
set contour base
set zlabel 'amplitude'
splot "graphics/plot.dat" matrix with lines
show output
s//----->\t/;$~="JAPH";s//\r<$~~/;{s|~$~-|-~$~|||s
|-$~~|$~~-|||s,<$~~,<~$~,,s,~$~>,$~~>,,
$|=1,select$,,$,,$,,1e-1;print;redo}
