In this case you have to use your own user-defined function in the nonlinear curvefit analysis object.
The circe function is (x-x0)^2+(y-y0)^2 = R^2
For the upper half of the circle you could use the following FPScript function:
Dim x0 = p
Dim y0 = p
Dim R = p
Sqrt(R^2 - (x-x0)^2) + y0