Surface gravity versus effective temperature for the models with diffusion only (left panel), and diffusion+rotation (right panel), both calculated for a metallicity of Z = 0.02. Each evolutionary track is plotted as a dot at 0.5 Myr intervals and colour-coded according to the value of log  (Ca/H) at the stellar surface. The over-plotted squares are the observed (proto)WDs with measured Ca abundances taken from Gianninas et al. (2014a).