k = 1.27 (slider from 0 to 5, step 0.01)
a = 1 (slider from 0 to 5, step 0.1)
b = 0.7 (slider from 0 to 5, step 0.1)
f(x) = a / (1 + b * exp(-k * x))
J: (log(b) / k, a / 2)