I instead used a workaround:
I housed the ellipse in a big transparent window so it can appear as though it grows from the center of the screen. However if you drag the ellipse towards the top of the screen, it gets pushed down (because windows OS always tries to fit the top of the invisible window into the screen instead of clipping it).
To overcome this issue, I used this thread: How do I move a wpf window into a negative top value?