Different photoionization models viewed in the P₁–P₂ diagram and their predicted relations between the metallicity and ionization parameter. Leftmost column: Different SF-ionized models and MaNGA MPL-7 data in the P₁ –P₂ diagram. The models were interpolated and cut so that only the parts that cover the middle 98% of the data along the hidden P₃ axis are shown. The spaxels with contributions from SF ionization greater than 90% are colored from yellow to green, while the rest of the spaxels are colored from white to black. Only the former were used to derive the metallicities and ionization parameters. Right columns (a) to (c): derived metallicities and ionization parameters using different SF-ionized models and different combinations of line ratios. In each panel, the green arrow marks the solar oxygen abundance adopted by the corresponding model.