Evolution of line luminosity ratios with time: Hα/Hβ (top), He II/Hα (middle) and He II/He I (bottom). The vertical axes in all panels are logarithmic. The Hα/Hβ ratio shows a large diversity within the sample and, occasionally, a significant time evolution for individual events. He II seems to be persistent in TDEs as the He II/Hα and He II/He I ratios do not drop with time. AT2018zr and LSQ12dyw do not show He II in their spectra hence we place upper-limits for those in the He II/He I ratio plot. For visual purposes, the large error bars in the He II ratios of AT2018dyb have been removed (see Sect. 5.1.2).