Showcase star formation + fast shocks hybrid models for the [N II] BPT. The upper panels correspond to the hybrid models with a contribution of fast shocks of 50% and 50% star formation. The lower panels correspond to 30% shocks and 70% star formation. The fast shocks models from Allen et al. (2008) correspond to shock velocities of 200–1000 km/s and magnetic fields of 10⁻⁴–10 μG cm^3/2, assuming shocks + a precursor (red curves) and only shocks (blue lines). The red arrow indicates the direction of increasing magnetic field in the shocks + precursor models, and the blue arrow indicates the direction of increasing shock velocity in the shock-only models. The dashed oblique line in the [N II]/Hα diagnosis represents the separation of shock excited (e.g. supernova remnants) from photoionised regions (e.g. H II regions) from Kopsacheili et al. (2020). The classic demarcation between H II regions, photoionisation, and AGNs from Kewley et al. (2001) is also plotted in all diagnoses. In every panel is plotted the hybrid models for a fixed metallicity and ionisation parameter: first and second row correspond to Z = Z_⊙ and Z = 2 Z_⊙, first, second, and third column correspond to q = 10⁷, 4 × 10⁷ and 2 × 10⁸ cm/s, respectively.