Not sure if this is in the right area of the forum (engineer not a mathematician) but here goes.....

I am trying to do some curve fitting and am trying the following sigmoid function (Gompertz function):

y(x) = a.exp[b.exp(c.x)]

Where 'a' is the asymptote to which y(x) rises to and 'b' (always negative) appears to shift the curve in the x direction and 'c' changes the charcteristics of the intial and end of the curve.

What I am after is a way of changing the gradient of linear section of the curve. Does that change with a change in 'c'? Or is there another variable needed?

Thanks :-)