Mass – and luminosity – H ii gas-phase metallicity relation of the XMP galaxies. The color code parametrizes the morphology as follows: red = symmetric, black = cometary, blue = two-knot, green = multi-knot and grey = no morphological information. Top: the symbol code is □= M_dyn, •= M_H   i and ×  = M_⋆. The figure includes the empirical correlation established for BCDs by Papaderos (2008; cyan line) and for green pea starburst galaxies (magenta line; Amorín et al. 2010). Bottom: the figure shows the luminosity – metallicity relation found for a sample of dwarf irregular galaxies (magenta line; Skillman et al. 1989) and for a sample of SDSS star-forming galaxies (cyan line; Guseva et al. 2009).