Ratio between the zeroth-order and higher-order coefficients of the GW anisotropy power spectrum. The distributions are obtained from 10⁵ sky realisations at a GW frequency of 1.98 nHz. The medians and means of each distribution are indicated by squares and circles respectively. The dashed black curve represents the NG15 upper bound (Agazie et al. 2023e), while other dashed curves indicate future prospects assuming EPTA-like noise and a 15-year observation time (Depta et al. 2025). The colours of the violin plots correspond to the same models as in Fig. 1.