Diagrams of [O III] λ5007/Hβ against [N II] λ6584/Hα (left), [S II] λλ6716,6731/Hα (middle), and [O I] λ6300/Hα (right), showing the location of an optically selected sample of AGN galaxies from our test sample (Sect. 4.1). The points are color-coded depending on their classification based on our diagnostic: AGN in green, SF in red, LINER in blue, and composite in yellow. We see that, since these are two-dimensional projections of the four-dimensional space used for the optical line-ratio classification, some AGN galaxies may fall outside the AGN demarcation line. The solid black curve is the extreme starburst line defined by Kewley et al. (2001). The straight black line is the separating line between AGN and LINER galaxies as defined by Schawinski et al. (2007). The dashed black curve is the Kauffmann et al. (2003) line separating SF from composite galaxies.