gnuplot is nice, but since you mention scientific, and you want a really professional one, have a look at the root package (
http://root.cern.ch), free, nicely supported, very professionaly done, nice tutorial, carried by a huge user base. I'm a scientist, and our community uses it as the swiss army knife from analysis to final professional-grade presentation and publication.
Here are some teaser pictures from the tutorial, you can actually download the scripts to generate those.
http://root.cern.ch/root/html/examples/gif/gerrors.gif
http://root.cern.ch/root/html/examples/gif/zdemo.gif
http://root.cern.ch/root/html/examples/gif/surfaces.gif
Have a look,
mlp
