I tried gnuplot a couple years back, found it unwieldy. I'll check it out again, plus the others mentioned. My brain aches already.
In other, unrelated news (hijack my own thread?), it took me all of 15 minutes to make a corner tapped orifice flowmeter using two pieces of copper pipe, an old penny, a straight pipe coupler, and a bit of small diameter brass tubing.
Now to see if it's of any use....