Corner plots showing the posterior probability distribution of key parameters for epochs 4 (left) and 5 (right) fitting as a P Cygni feature from the set of Y⁺ lines from the 4d5p − 4d² transitions with λ ≥ 700 nm. The fit parameters are (1) the cross-sectional velocity, v_⊥, derived from the blackbody normalisation fit assuming Planck cosmology (Planck Collaboration VI 2020), (2) the photospheric velocity, v_phot, (3) the maximum outer ejecta velocity, v_max, (4) the velocity range, Δv = v_max − v_phot, and (5) the average optical depth of the Y⁺ lines weighted by their relative strength, τ_avg. Velocities are given in units of the speed of light, c. The central wavelength of a line, λ₀, and its relative strength is set by the 4d5p to 4d² transitions in Y⁺ (see Biémont et al. 2011 and Table 2) in LTE at about 0.4 eV. These constraints show only the statistical uncertainty in the fitting model.