I don't think I quite got there, but maybe this can give ideas.
Instead of using the stock shapes, I plotted the points of a circle
using the method described inKen Flerlage's excellent post:
I made a sheet with two columns: Type (Radiation/Baseline) and Point (increasing integers to 37).
Then X became:
IF MAX([Type])="Baseline" THEN COS(SUM([Angle in Radians]))*SQRT([baseline value]/PI())
ELSEIF MAX([Type])="Radiation" THEN COS(SUM([Angle in Radians]))*SQRT(SUM([Sheet1 (Book1)].[Global Horizontal Radiation])/PI())
And Y the same except with SIN.
The graph was resizing such that the baseline always filled the grid,
so I fixed the axes to allow the baseline size to change with the parameter.
Please see the workbook v10.3 attached in the Forum Thread.