Is it possible to use the Curve Fitting Toolbox to fit a parametric curve to a shape?

I am attempting to fit a parametric curve to a set of data that is circular (not a Cartesian function). It seems from the documentation that this should be possible if I create functions for x and y in terms of arclength, but when I try to use the toolbox I get results that go straight through the circle, not around it. I feel like there is something simple I am missing.

If someone who knows this toolbox well could offer advice it would be most appreciated.

[L K]

Hi, did u solve this problem of yours? Actually even i have to fit a curve on a polar plot ...circular...please can you help me..please