PLplot is a library of C functions that are useful for making scientific
plots from a program written in C, C++, or Fortran.  The PLplot library
can be used to create standard x-y plots, semilog plots, log-log plots,
contour plots, 3D plots, mesh plots, bar charts and pie charts.  Multiple
graphs (of the same or different sizes) may be placed on a single page
with multiple lines in each graph.  Different line styles, widths and
colors are supported.  A virtually infinite number of distinct area fill
patterns may be used.  There are almost 1000 characters in the extended
character set.  This includes four different fonts, the Greek alphabet and
a host of mathematical, musical, and other symbols.  The fonts can be
scaled to any desired size.  A variety of output devices are supported and
new devices can be easily added by writing a small number of device
dependent routines.

WWW: http://plplot.sourceforge.net/