Opacity of each τ-bin in pairs of λ-ranges for the four stars (spectral type at the top left of each panel) versus the geometrical height (bottom axis of each panel) and optical depth (top axis). The λ-ranges are selected to illustrate the different variations of the opacity with height depending on the spectral region. The colours of the curves correspond to the λ-ranges in Fig. C.1, with the values of each range in the legend.