S-Shaped Trendline - Please Help

Hey all,

I'm trying to write a VBA code to be able to fit an S-Shaped trendline through a scatter plot in excel. The Curve must be of the form Y = a*atan((x/b)+c)+d.

I've tried estimating the A,B,C,D through Solver, but it takes too long and oftern doesn't find the best solution.

Do i need to go into writing my own VBA for solver, or is there a better way?

I've attached an example of what i'm working on.

Kind Regards

Sam