This sets the the size of the pie chart in viewport space by defining the circle diameter in relative units to the viewport size. The size of the whole chart incl. chart title text, segment names and boundary square is bigger than this and gets dynamically scaled. This is only used when drawing in screenspace via DrawScreenspace(), but ignored when drawing via Draw(). Viewport space goes from lower-left (= 0,0) to upper-right (= 1,1).
The name "*_relToCamViewport" is ambiguous, because it leaves open if the width is relative to the camera viewport height or the the camera viewport width. This decision is made via the optional "chartSize_isDefinedRelTo_cameraWidth_notCameraHeight" parameter of the DrawScreenspace() function. The default is "relative to viewport width".