Core-to-continuum median flux ratio versus significance of the Spearman correlation for all quasar spectra searched in a proximate velocity window (top, within a velocity window encompassing the pipeline and visual redshifts estimates, extended 2000 km s⁻¹ on each side) and an intervening window with the exact same width for each spectrum, but shifted by 5000 km s⁻¹ bluewards (bottom). The vertical and horizontal dotted lines show our cuts defining the samples (top) and (bottom). Points located outside the solid contour (containing 99.73% of the points) define, respectively, (top) and (bottom). Candidates belonging to either one or both of these selections (black points) were visually checked and coloured green when strong H₂ is confirmed (grade A) or yellow when considered tentative only (grade B). Red and orange points correspond to additional systems described in Sect. 2.4 with, respectively, grade A and B.