BPT showing the mixing lines (solid grey lines) between AGN and H II region models. Each panel is for a single H II region mixed with AGN models of a given SED (indicated on the top left) spanning all values of U from our grid of models. Points on the mixing lines mark η = 20, 40, 60, 80 and 100% and are colour-coded by the values of L_bol^mod/L_Hα. Background points in cyan are galaxies from the MGSz sample and in blue are OPARGS galaxies; curves in black are the Stasińska et al. (2006), Kauffmann et al. (2003) and Kewley et al. (2001) lines.