Gnuplot is a command-driven interactive function plotting program.

Plots any number of functions, built up of C operators, C library
functions, and some things C doesn't have like **, sgn(), etc.  Also
support for plotting data files, to compare actual data to
theoretical curves.

User-defined X and Y ranges (optional auto-ranging), smart axes
scaling, smart tic marks.

Labelling of X and Y axes.

User-defined constants and functions.

Support through a generalized graphics driver for AED 512, AED 767,
BBN BitGraph, Commodore Amiga, Roland DXY800A, EEPIC, TeXDraw, EmTeX,
Epson 60dpi printers, Epson LX-800, Fig, HP2623, HP2648, HP75xx, HPGL,
HP LaserJet II, Imagen, Iris 4D, MS-DOS Kermit, Kyocera laser printer,
LaTeX, NEC CP6 pinwriter, PostScript, QMS QUIC, ReGis (VT125 and
VT2xx), SCO Xenix CGI, Selanar, Star color printer, Tandy DMP-130
printer, Tek 401x, Tek 410x, Vectrix 384, VT like Tektronix emulator,
Unix PC (ATT 3b1 or ATT 7300), unixplot, and X11.  The PC version
compiled by Microsoft C supports IBM CGA, EGA, VGA, Hercules, ATT
6300, and Corona 325 graphics.  The PC version compiled by Borland C++
sup- ports IBM CGA, EGA, MCGA, VGA, Hercules and ATT 6300 graphics.
Other devices can be added simply, but will require recompiling.

Shell escapes and command line substitution.

Output redirection.

Eventual modifications to the source are in 
${PREFIX}/share/gnuplot/4.0/patches