Diagrams involving two ratios sensitive to the ionization degree. Top: [O iii]λλ4959, 5007/[O ii]λλ3726, 3729 vs. [O ii]λλ3726, 3729/Hβ. Bottom: [S ii]λλ6717, 6731/Hα vs. [O iii]λλ4959, 5007/ [O ii]λλ3726, 3729). The locus of equal estimated ionization parameter according to the relations proposed by Díaz et al. (2000) and assuming Z = 0.3   Z_⊙ is indicated with a black line. Only those spaxels with estimated uncertainties for the involved ratios smaller than 0.5 dex have been taken into account. Spaxels were grouped in three bins according their n_e([O ii]) as indicated in the upper right-hand corner of the individual diagrams. The total number of considered data points is indicated in the lower right-hand corner.