Index–index diagnostic diagrams, on the left Hβ − Mg_b and on the right Hγ_F–[MgFe]’. The points are colour-coded by their literature value of [Fe/H]. We report TMJ models as a grid with varying metallicity (vertical lines) and varying age (horizontal lines), the first with the same colour code as the data points. Solid lines refer to models with [α/Fe] = 0.3, while the dashed lines in the background represent the solar-scaled grid.