I  never used MATLAB before, so I don’t know how powerful it is.  In order to plot graph and perform calculation, these are some applications may be interesting,

• R - It is also called R programming language, used for statistics
• Octave - Alternative to MATLAB
• FreeMat - Alternative to MATLAB
• Sage - A large size application, considered as an alternative to Mathematica and MATLAB

Another one I like is Python with SciPy and matplotlib. They are pretty awesome.