Upper panel: observed source-frame spectra stacked over different observational periods after subtraction of power-law continuum. Middle panel: differences between spectra stacked over limited observational periods and the spectrum stacked over all available observations (line styles correspond to those in the upper panel). Lower panel: model for the spectral decomposition of the Mg II line and theoretical Fe II pseudo-continuum template. The spectral windows for continuum evaluation are indicated with grey stripes.