(Left) R₃₂-vs.-R₂₁ diagram for the galaxy NGC 4569. The x-axis represents R₂₁ and the y-axis R₃₂. The intensity scale gives the sky area in the map per unit area in the ratio space, it is expressed in beams per 0.01 × 0.01 ratio area. This diagram has been produced by convolving the actual distribution of pixels in the ratio-ratio plane by a Gaussian kernel with a FWHM of 0.02. Note that the pixels in the ratio maps used to compute the distribution are much smaller than the beam (see Sect. 4). The contour levels are 0.3, 0.6, 0.9, 1.2 and 1.5 beams/0.01². (Middle) R₃₂-vs.-R₂₁ diagram for the galaxy NGC 4826; the contour levels are 0.3, 0.5, 0.7, 0.9, 1.1 beams/0.01². (Right)LTE and pseudo-LTE models represented as black curves in the R₃₂-vs.-R₂₁ diagram. Curves 1 and 2 correspond to pure LTE models with N_CO/dV = 10¹⁵ cm^-2 pc (km s^-1)^-1 and N_CO/dV = 12 × 10¹⁵ cm^-2 pc (km s^-1)^-1, respectively. The temperature increases from 4 K at the bottom left to 8 K at the top right. Curves 3 and 4 correspond to pseudo-LTE models with f₃₂ = 0.5 and f₃₂ = 0.2 respectively (see Sect. 5.1 for the definition of f₃₂). The column density to line width ratio is N_CO/dV = 10¹⁵ cm^-2 pc (km s^-1)^-1 and the temperature increases from 4 to 20 K along the curves from bottom left to top right. The cyan and magenta curves show the lowest contours for NGC 4569 and NGC 4826, respectively.