Parametrisation of the T2-T3 plane from Yates et al. (2020). MARTA galaxies are overplotted, colour-coded by their T_e-based metallicity. The solid lines represent the T-T relation calibrated by Yates et al. (2020) for a fixed metallicity. Inset panel: ΔZ vs Z plot, where Z is the metallicity derived from the T_e method for the MARTA objects and ΔZ is the difference between the ‘direct’ metallicity and that obtained from the Yates et al. (2020) calibration for the same galaxies.