I don't know what exactly you want/need but there are a couple of options:
Works cross platform and is command based. There are many options for both 2D and 3D graphs. Just take a look at the demos for Gnuplot.
Gnuplot is a portable command-line driven graphing utility for Linux,
OS/2, MS Windows, OSX, VMS, and many other platforms. The source code
is copyrighted but freely distributed (i.e., you don't have to pay for
it). It was originally created to allow scientists and students to
visualize mathematical functions and data interactively, but has grown
to support many non-interactive uses such as web scripting. It is also
used as a plotting engine by third-party applications like Octave.
Gnuplot has been supported and under active development since 1986.
Works with python and might be better for starting of with command based plotting. There is a tutorial site and as with Gnuplot a page of examples. There are more options for graphs than you could propably think of.
Desktop aplication for KDE. Can do simple 2D graphing and some more advanced things like derivatives and integrals. It is simple to use but not nearly as capable as the Gnuplot and matplotlib. Does not work cross platform. Really only useful for getting a png or jpg of a graph and to do some school level calculations.
Works both in the browser and as desktop application. Has an option to sync you projects in the browser. Works for simple 2D and 3D Graphs but doesn't come close to the former two in features. Mostly usefull in the classrom or to replace a graphing calculator.
Very similar to Geogebra in the browser with similar usecase. Can only be used in the browser and only for 2D graphs. Really just for school and as an online graphing calculator. Also has some other useful tools like the scientific calculator.
If you are already familiar with matlab-like commands you should definitely go with Geogebra or Matplotlib. The former is more common in universities (at least where I live) the latter might be nicer to get started with. If you just need a quick 2D graph I would go with Geogebra.