Example of fit of the MgII line for SDSS J0246-0825 (black solid line) with a model (red) using the procedure described in Sect. 3.1. The model is the sum of a power-law continuum (blue), a Fe II template (green), a broad Mg II component (dotted gray) and a narrow Mg II (dashed gray). The gray crosses indicate the region of the spectrum affected by atmospheric absorption and not used for the fit. The dashed black line shows the residual of the fit (data-model).