Fraction of white dwarfs with calcium absorption (ƒ_Ca) as a function of the effective temperature (T_eff). The best-fit F_Ca model and its 68% confidence interval are shown with the purple line and area, respectively. The parameters of the linear trend from Eq. (20) are presented in the panel. The purple dots depict the fractions obtained with the temperature-independent model D₂ at different T_eff intervals.