Graphic definition of the quantities used for comparing Lyα flux. The spectrum of a z​ ∼ ​2.2 QSO from SDSS DR14 is used as example in all panels (grey lines). Each Lyα flux definition is outlined by a grey shaded area (see text for details). Yellow lines in each panel show the reconstruction of the source continuum (power-law and linear approximation, respectively, in the first two and last two panels from the top). Finally, coloured squares and crosses (third and last panel from the top, respectively) show J-PLUS measurements and synthetic photometry, respectively, performed on the SDSS spectrum with J-PLUS transmission curves (Eq. (A.2)).