Contribution of the most important lines to the logarithm of the H₂ column density around the mean column density of the training set as a function of the visual extinction. One red points per pixel of the test set is plotted. The black histograms show the median values of all data points falling in a regularly sampled interval of the logarithm of the visual extinction. The black error bars show the range of values where 50% of the points in the current bin are located. The vertical dotted lines show visual extinctions of 1.1, 2.2., 5.0, 10.0, and 18.0. Except the ¹³CO (1–0) line that contributes at all A_v and the noise sample that does not contribute at any A_v, the other lines contribute inside a given A_v range. The lines are sorted from top to bottom and then from left to right by increasing values of the minimum A_v at which they start to contribute.