I do use and recommend http://desmos.com, which is proprietary (but pretty limited and easily translated to Python). It doesn't lock features behind a paywall like Wolfram Alpha.
Desmos looks interesting.
For example:
Quadratic through 3 points (drag the points around): https://www.desmos.com/calculator/tf1f80zgug
Pythagoras's Theorem (drag the sliders): https://www.desmos.com/calculator/dbjxbeuzk7
Why shearing preserves area (drag the slider): https://www.desmos.com/calculator/1vykaqf7je
If one doesn't mind proprietary, though, and wants even more capable and controllable, Mathematica on a Raspberry Pi is pretty cool, and is free (beer) and is included in the default install of Raspbian.
I also use python/sympy/matplotlib/scipy/jupyter a lot and it's absolutely great -- highly recommend to the OP.
I think if you're building up skills and knowledge, it should be towards something you have control over and can use in any situation without worrying about licences, cost, etc. At my university, we teach undergrad engineers Matlab, and it just seems like an expensive clunky dead end to me (though their numerical methods knowledge should be transferable).