Stack of spectra with detected Mg II absorption (top panel), emission (middle panel), and P-Cygni (bottom panel) profiles. The black lines represent the stack of all the galaxies of each class relative to the systemic redshift inferred from other ISM lines. The filled gray spectra instead refer to stacks on the minimum of the K line for the absorption sample and the maximum for the emission for the emitters and the P-Cygni samples.