Correlations between the C¹⁸O 1 → 0/H(41)α, CN 1 → 0/N₂H⁺1 → 0, and CS 3 → 2/N₂H⁺1 → 0 brightness temperature line ratios along the galactic plane shown in Fig. 5. The plots are in logarithmic scale. To obtain a more reliable result that avoids the line borders, we only considered the points within the following range of values: 0.5 and 5 for C¹⁸O 1 → 0/H(41)α line; 5 and 15 for CN 1 → 0/N₂H⁺1 → 0; 1 and 6 for CN 1 → 0/CS 3 → 2 between 1 and 6. The results of the least-squares fitting and the values of the Pearson correlation coefficients are shown in the top and middle panels.