Probably you need an 'oval' rather than a 'circle'.
Open the 'Big Class' sample data, then run the Bivariate script it contains. Right click on the graphics box, and select 'Customize...'. Click the '+' sign to add a new graphics script, and enter:
Oval(55, 80, 60, 120);
and then 'OK'. This should give an idea of how it works.