Colour–magnitude diagram (top) and colour–colour diagram (bottom) of the DSC training data for the quasars (blue) and galaxies (orange) as well as stars (black). The contours in each panel show the variation in source density on a linear scale. The points are equal-sized random subsets of sources from each class. There is significant overlap, in particular between stars and quasars: in reality the former dominate by a factor of about a thousand, and so overlap much more than is shown here. Plots for each class separately are provided in the online documentation.