Selection of 21 galaxies from the 12+log(O/H) versus SFR diagram. Left panel: Relation between 12+log(O/H) and SFR for star-forming galaxies. All the lines have the same colours as in Fig. 2. A sample of 21 galaxies has been selected (as an example) in this diagram; their positions are shown with grey circles and have been labelled from 1 to 21 according to their SFR. Right panel: Relation between SFR and M_⋆ for star-forming galaxies. The points are colour-coded according to their D_n(4000): blue if D_n(4000) < 1.2, and red if D_n(4000) ≥ 1.2. The dashed lines represent the 1σ, 2σ, and 3σ contours in each D_n(4000) subsample. Grey circles show the positions in the SFR–M_⋆ diagram of the 21 galaxies.