Spectral composite of the L20 sample. Top panel: composite spectrum of our cosmological sample described in this work. The Lyα emission line is cut for visualisation purposes. The inset zooms in on the Hβ–[O III] region. The main emission lines are labelled accordingly. Middle panel: relative residuals with respect to the VB01 template. Bottom panel: number of objects contributing to each spectral channel. The composite spectrum of the L20 sample is available in its entirety in a machine-readable form at the CDS.