Measurements of H¹²CO⁺/H¹³CO⁺ as a function of R_GC and comparison of the derived gradients between different works. Blue dots represent τ_HCO+ < 3, and black dots represent τ_HCO+ > 3. The two lower limits are shifted to R_GC = 0.5 kpc for better visualization. The red curve denotes the maximum likelihood fit of the blue points only; orange curves denote the 3σ deviation from the MCMC sampling.