Galactocentric gradients of column densities, N_tot. The plots show the trends for HCN (a), HCO⁺ (b), c-C₃H₂ (c), H¹³CO⁺ (d), HCO (e), SO (f), H₂CO (g), and CH₃OH (h) as a function of Galactocentric radius (R_GC). The upper limit values are represented with triangles. The dark blue lines represent the linear regressions computed over the datasets. In the upper-right side of each subplot, the Pearson correlation coefficient, ρ, is shown.