In my opinion there isn't really anything that competes with MATLAB for the things it is really good at. Numpy is shit.
If you really can't afford £120 for MATLAB then I would say the next best option is Scilab.
Now we just need SOLIDWORKS to introduce a decent hobby license. They added one recently but it's subscription only so screw that.
Are you sure? The standalone version of Microsoft Office is more expensive at €150.
Windows is even more expensive, but of course most people get that bundled with their computer.
Wait, what?
I switched all my plotting in my thesis (5 years ago) from matlab to octave precisely because matlab plots were terrible.
Are you thinking of octave v3 from 15 years ago when it relied on gnuplot by any chance?