qmatplot is a gnuplot-like tool for plotting data sets in either two or three dimensions. It can plot many types of 2d plots, including pixmaps and contours, and 3d surfaces. qmatplot makes available a full WYSIWYG mode along with multiple objects on the same page.
As opposite to Gnuplot, qmatplot is very clickable, so it might be easier to use for unexperienced users. It can be used instead of Gnuplot with Octave and Scilab - one can compile and install new function libraries installed in those packages, which communicate with qmatplot through an Unix socket.