Left panel: EW_abs, Hδ against r-band brightness (left axis for flux density units, f_ν; right axis for absolute magnitudes, M_r) from the best-fitting results of the subsample with flux-calibrated spectra. The colour code marks the systems associated with each data point. A dashed black line depicts the best linear fit to the correlation, and dashed cyan, dashed red, and dashed black lines are the equivalent for individual datasets. Right panel: EW_abs, Hβ against EW_abs, Hγ from the best-fitting results of the whole spectroscopic sample. The colour code marks the systems associated with each data point, while symbols differentiate between subsets as described in the legend. A dashed black line depicts the best linear fit to the correlation, which is consistent with a 1:1 ratio. Transparent symbols correspond to fitting parameters that are consistent with null BA within 3σ.