i only found out today the answer to my question. although no one has answered it i guess i will share it for the benefit of everyone.

You have to define the circle as a parametric equation by clicking on graph->new parametric function, then a circle is defined by x=cos(t) and y=sin(t)

or you can define the circle on a polar graph and by clicking on graph->new polar graph, and type any number for r this will give a circle of radius r.