Excel 2008 Chart Trouble

I am trying to create a smooth lined xy plot where I know the equation for the given function. I am trying to essentially graph an ellipse for a Calculus class and have had no success in being able to do so. I begin running into trouble after I define terms, such as x and y, for the sheet and then attempt to add those defined terms into the Series portion of the chart, it seems to only be adapt to selecting terms in the given cells, however I don't need to figure out every coordinate since I know the equation... if anyone has any idea what I'm talking about please help...