Dust and ¹³CO line emission as functions of column density of the large-scale ISM simulation. The blue 2D histogram shows the distribution of the ¹³CO line area vs. the true column density. The colour bar indicates the number of pixels in the original line emission maps per 2D histogram bin. The corresponding distribution of 250 μm surface brightness vs. column density (right-hand axis) is shown with red contours. The contour levels correspond to log₁₀n_pix = 0.5, 1, 2, and 3 for the number image pixels n_pix per 2D histogram bin. The number of 2D bins is the same as for the 250 μm surface brightness histogram.