Example of full-spectral fitting for one of the M31 spectra. The four panels correspond to the spectral ranges defined in Sect. 4. In each panel, the upper subplot shows the observed spectrum (black) and the best-fitting model (red), both normalized by the best-fitting multiplicative polynomial, while the lower subplot shows the relative residuals (observed minus model), the ±1-sigma uncertainties (gray shaded regions), and the best-fitting model for the emission lines (blue curves).