Comparison of the average metallicity properties of our sample of dwarf galaxies. Top left: Stellar luminosity-metallicity diagram; MW (M31) satellites marked with blue circles (orange squares), while isolated systems are shown as red triangles; the linear relation recovered for our sample is indicated by a dotted line and a rms band in light grey, while a dashed line and rms band in dark grey indicate the Kirby et al. (2013b) relation. Top right: Same as the left panel, but with symbols indicating systems whose ⟨[Fe/H]⟩ have been obtained with the CaT calibration method (orange squares) or using a spectral fitting technique (blue circles); the linear relation recovered for the sub-sample using the CaT method is indicated by a dotted line and a rms band in light grey. Bottom left: Difference between the weighted-average ⟨[Fe/H]⟩ of our galaxies and those obtained from the Kirby et al. (2013b) relation at their luminosity; symbols as in the top right panel. Error bars were obtained adding the ⟨[Fe/H]⟩ errors and the rms = 0.16 dex of the Kirby et al. relation in quadrature. The dashed line is the median offset of 0.02 dex for the sub-sample using the CaT-method. Bottom right: Difference between the weighted average and the median (mode) of [Fe/H] distributions marked with blue squares (orange triangles) as a function of luminosity. The dashed (dotted) lines indicate the rms scatter.