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gnuplot plots 2d and 3d graphs, from a data file or with a formula. It has an interactive mode with online help, or it can be used non-interactively. gnuplot does function fitting to data sets, and it does output to many terminals, among which are PostScript, X11 display, PNG, and GIF (via the old gd library).
Octave is a high-level language, primarily intended for numerical computations. It provides a convenient command line interface for solving linear and nonlinear problems numerically, and for performing other numerical experiments. It may also be used as a batch-oriented language.
Freemat is an intepereted, matrix-oriented development environment for engineering and scientific applications, similar to the commercial package MATLAB. It provides visualization, image manipulation, and plotting as well as parallel programming.