Sulphur (left) and silicon (right) line regions. The spectrum of Obs. 3814 is shown relative to the linear fit from Fig. 2 (black) and compared to the count spectrum (lower panels) from Hirsch et al. (2019) and the rest-wavelength of detected lines. The count spectrum is given relative to a local power-law fit in the wavelength bands indicated in the figure and is the sum of spectra of all dip stages. Two time binnings are displayed for the spectrum, 50 s (blue) and 500 s (red), to investigate the effect of the added higher frequencies. Lines connect the data points for clarity. Overall, there is a good correspondence between spike features and known line positions.