There are certainly cleaner ways to do this other than the brute force method used here,
but maybe this can be a starting point for you.
I have a data source with columns x and y and type,
type being either data or circle.
They are plotted on a dual axis, line for circle and scatter for data.
The check for whether in or out of circle is:
ABS([DataX])<.005 AND ABS([DataY])<.005
The circle in the spreadsheet is x values running from -.005 to .005 and back down to -.005
and then y being the squareroot of r squared minus x squared (and the negative version for the part
where x returns back to -.005)
(in the attached, I used a radius of .005, and changed the example points to match)