GNU 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 using a language that is mostly compatible
with Matlab.  It may also be used as a batch-oriented language. 

Octave has extensive tools for solving common numerical linear algebra
problems, finding the roots of nonlinear equations, integrating
ordinary functions, manipulating polynomials, and integrating ordinary
differential and differential-algebraic equations.  It is easily extensible
and customizable via user-defined functions written in Octave's own
language, or using dynamically loaded modules written in C++, C, Fortran,
or other languages. 

In order to use the plotting commands, you must also install gnuplot.