Plasma diagnostics using PyNeb. The dashed or dotted values indicate the density-temperature loci corresponding to the observed reddening-corrected line ratios (also aperture-corrected when combining Spitzer and optical data, as explained in Sect. 4). The coloured bands delineate the one sigma error zones in the corresponding ratios. Diagnostics involving oxygen ions are in green: [O ii]na 3727+/7325+; [O ii]n: 3729/3726; [O iii]na: 5007/4363. Diagnostics involving sulfur ions are in black: [S ii]n: 6716/6731; [S ii]na: 6720+/4072+; [S iii]fn: 18.7 μm/9069; [S iii]na: 9069/6312; the [N ii]na 6584/5755 diagnostic is in blue; the [Ne iii]fn 15.6 μm/3869 diagnostic is in pink; finally the [Fe iii] 5272/4987 diagnostic is in blue. (The characters n, a, f in the denomination of the line ratios stand for “nebular”, “auroral” and “fine-structure”, respectively).