Fraction of leaking Ly-α photons at the core of the DLAs as a function of its relative velocity to the quasar redshift. Filled points correspond to systems from the two statistical selections described in Sect. 2 (flag ≠0 in Table 1). Unfilled symbols correspond to the additional systems described in Sect. 2.4 (flag = 0). The colour indicates the visual classification (black:A, grey:B). Finally, red squares are overplotted on top of systems with clear Si II^* absorption. The solid (resp. dashed) segments correspond to the median (resp. mean) values in different velocity bins, using only statistical rank A systems. Values measured to be less than 0.01 are set to 0.01 for plotting convenience. The cross at the top-left corner shows typical (albeit conservative) uncertainties along both axes.