Diagnostic graphs for our four characteristic spectra. Red: total, green: blue spot, blue: middle peak, cyan: the rest (see Table 1). The line ratios from Table 2 are compared to the HII-region models from Dopita et al. (2013) (for a Boltzmann distribution of electrons) with the ionisation parameter log q and three oxygen abundances in solar units (1×, 2×, 3×) as parameters. The solid lines are lines of constant abundance and varying log q. The range of log q values is indicated in the left panel, which is the same for all panels. The dotted line in the middle panel is the line separating HII-regions from AGN-like spectra according to Kauffmann et al. (2003). As noted by Dopita et al. (2013), the right diagnostic graph seems to provide the cleanest separation of log q and oxygen abundance. Spectra 2 and 3 (blue spot and middle peak) show the characteristics of a HII-region and a super solar metallicity in all graphs. The other spectra show contributions from other ionising sources, presumably post-AGB stars. The right panel is apparently insensitive to this circumstance.