Absolute (top panel) and relative (bottom panel) difference of derived plage areas from images with imposed disc ellipticities to those from the original Rome/PSPT data. Shown are the annual median values (dashed lines), the asymmetric 1σ interval (shaded surfaces), and the annual maximum and minimum values (solid lines). The colours denote results for different disc eccentricities between 0.025 (dark blue) and 0.5 (red).