Signal plotting betweend two points

Hello guys.

I need your help. What function should I use to plot that part of the signal from fmin to fmax in the figure below. I am thinking to split that part from fmin to fmax in two different parts and use an exp function for the first part and a log function for the second part. What do you think ? Thank you.

Answer 1

[madhan ravi]

Piecewise