Evaluations of Free Scientific Plotting Software?

paulywog asks: "I've used a fair number of non-Free (Beer) scientific plotting packages during college and as an engineer -- including Matlab, SigmaPlot and DeltaGraph, Excel (ick), SAS, my trusty HP 48G, among others. Lately, I've been looking for OSS resources that can provide similar features. Part of the reason is that there are some features that I've wanted, but never been able to get out the other software. One feature in particular: shade the area between two curves or shade the region outlined by error bars on this plot. I'd like to hear what experience other scientists and engineers have with OSS plotting packages in terms of ease of use, power, and flexibility."