Flux-normalised spectral traces of the star β Pic, for one single spectral chunk. The panels are not plotted as pixeled images (which would not be possible for the bottom panel, since it is not uniformly sampled), but as one coloured dot per sampled data point. Yellow is a normalised flux of 1, blue is 0. Top panel: flux sampled by the x_i and y_i coordinates on the detector. Bottom panel: the same flux (no interpolation) sampled by wavelength λ_i and separation ρ_i.