[O iii]/[O ii] vs. [Ar iv]/[Ar iii] diagram for ionization-bounded photoionization models with a blackbody ionization source (12 + log O/H = 8). Top panel: models for filled spheres; bottom panel: models for spherical bubbles. Continuous lines correspond to different values of the effective temperature, while dashed lines correspond to different values of the ionization parameter. The color coding is indicated in the plot. The positions of the observational points for our samples (red for DR10, blue for DR7) are shown for comparison. This diagram shows that for a given value of [O iii]/[O ii] the value of [Ar iv]/[Ar iii] increases with T_eff.