Modelling of the wiggles in single-spaxel spectra. Top panel: integrated LBQS 0302−0019 spectrum (orange curve), single-spaxel spectrum (blue), and wiggles (grey) as already reported in Fig. 5. The red curve represents the best-fit model of the wiggles. Central panel: single-spaxel spectrum after the correction for the wiggles (dark blue), in comparison with the integrated spectrum (orange); the grey curve represents the new residuals with respect to the integrated spectrum. Bottom panel: best parameter for the frequency of the sinusoidal functions used to model the wiggles (blue points); a low-order polynomial function fitting these points is also reported. All panels display red shaded regions (associated with the QSO emission lines) that are excluded during the fit.