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Do you want the value to grow at a constant rate? Use a linear function.
Do you want the value to grow slow at first, but fast later? Use a polynomial
or exponential function.
Do you want the value to grow fast at first, and slow down later? Use an
nth-root or logarithmic function.
Do you want the value to grow slowly at the start/end but fast in the middle?
Do you want it to grow quickly but still have an upper-bound? Use a
sigmoid curve, such as atan or the logistic function.
Do you want the value to oscillate? Use a sin or some other wave.