grace
A few of its features are: User defined scaling, tick marks, labels, symbols, line styles, colors, polynomial regression, splines, running averages, DFT/FFT, cross/auto-correlation, batch mode for unattended plotting, and hardcopy support for PostScript, FrameMaker and several image formats.
Latest reviews
The best general purpose graphing and charting package I have come across. Not only does it produce publication-quality graphs and charts, but it takes care of details other packages don't even think about. Documentation is incomplete, but it is still easy to learn.
Excellent 2-D charting tool, capable of producing publication-quality charts that are customizable to the slightest details. I have used it for years, and I actually used it for every chart in my dissertation. It has a very decent GUI and is very interactive, which is something Gnuplot lacks. It does have a conterintuitive interaction at times, but one gets used to it in short notice. Too bad it is not being actively developed anymore, but its robustness and power grant it a place in one's toolbox and in free software repositories.