Integrated photon flux with respect to point-like emission at the center of the ROIs versus the number of co-added ROIs. The stacking is performed separately for three cases: simulated point-like sources (PS) with an integrated flux of 7.5 × 10^-10 ph/(cm² s) (dashed line) are added to simulated diffuse background regions, the first sample of dark patches that contain as few sources as possible and to the second sample of dark patches that contain Fermi-LAT sources with an angular separation between 5° and 10° from the ROI center.