Relation between the Hα luminosity and O/H, at left, and N/O, at right, for the selected H ii regions of the galaxies for which a linear gradient was calculated. The colour encodes the total stellar mass of the galaxy where each H ii region is situated. The histograms show in logarithmic scales in all axes the distribution for all objects represented in the plot (white bars) and for those H ii regions in a galaxy with an inverted gradient (black bars). Colours indicate the density of points and solid lines represent the 1σ, 2σ, and 3σ contours.