SPL fit of baryon mass fraction f_bar in IllustrisTNG at R_500c for all redshifts. Black dots show individual haloes; the green region indicates the SPL fit. Each panel is labelled with their corresponding simulation tag and the value for the α_SPL slope from the SPL fit in the upper-left corner. Top panels: fraction of baryons includes the hot gas and the galaxy stellar mass within the $2\times R_{M_{\mathrm{star}}/2}\sim r_{\mathrm{op}}$ aperture. Bottom panels: fraction of baryons includes all the gas and all the stars inside the halo. The yellow areas represent the Planck Collaboration XIII (2016) fraction of baryons. The C18 data (blue dots) is also shown, with its SPL fit (blue areas). The shaded areas are represented with a width of 3σ.