It turns out this post has been getting hits. In case you want to know how to make gnuplot output images, here is my recipie:
set term png small size 500, 500
set output "output_file_name.png"
plot "my_input_data" using 3:6 title "hello gnuplot image"
This creates a 500px square image (I think the “small” might be redundant having specified the size), with an x axis range of 50, using columns 3 and 6 of a tab delimited ascii source file.
I thoroughly recommend the following gnuplot infrequently asked questions. It contains instructions for doing both normal, and unusual things in gnuplot, in particular how to use a shell script with sed and awk to automate the plotting of an arbitrary number of images.