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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).
This is another library for creating and manipulating graphs. There is simply no other graph library out there which can be handle graphs of the size the author is confronted with efficiently. Whenever possible igraph tries to be also user friendly
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and portable.
igraph started as an additional package to the GNU R statistical environment, and still some functions which are hard to implement in C are available only in R (like interactive graphics). Most functions are however now written in C and they can be compiled without R as a separate library. [Less]
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.