I think these curves are meant only for animations. For game logic curves, you should choose an elementary function (https://en.wikipedia.org/wiki/List_of_mathematical_functions...) that fits your curve best and implement the math yourself. For an EXP-needed-to-level function, it would be simplest to implement an eponential curve (https://en.wikipedia.org/wiki/Exponential_growth) of the form c⋅b^x. In your game, that would be (initial_exp ⋅ (growth_rate ^ level_number)). Or you could implement the quartic function (https://en.wikipedia.org/wiki/Quartic_function), which represents the easeInQuad curve you liked on the web page.