Fraction of observed He⁺ ionising photons compared to the Xiao et al. (2018) model expectations as a function of observed He II λ1640 luminosity of the MUSE He II λ1640 sample. From left to right, we show the He⁺ model predictions computed for three metallicities, Z = 0.01, 0.001,  and 0.0001 with log₁₀(n_H)=1.0 and U_s = −1.5 for different times between 1 and 20 Myr from the onset of the star burst. The dashed horizontal line indicates y = 0, where there is no difference between observations and model predictions.