From the website:
Mathics is a free, general-purpose online computer algebra system featuring Mathematica-compatible syntax and functions. It is backed by highly extensible Python code, relying on SymPy for most mathematical tasks and, optionally, Sage for more advanced stuff.
A general mathematics package that self-describes some of its needs as folows:
Apart from performance issues, new features like 3D graphics and more functions in various mathematical fields like calculus, number theory, or graph theory are still to be added. (http://www.mathics.net/doc/manual/introduction/what-is-missing/)
As you explore graphs and other structures, you might want to consider contributing to this project.