Four main classes for the subset of 302 214 spectra. In all similar plots in this paper, the black line indicates the mean spectrum of each class, and the grey zone lies between the 10% and 90% quantiles for that class. The vertical scale is arbitrary and identical for all spectra. The class index and the number N of objects in the class are given in the legend. Classes are organised from left to right, top to bottom, in decreasing order of number of spectra.