3.2 × I_HCN/I_CO as a function of the fraction of gas above (from left to right) n ∼ 10^2.5,  10^3.5,  10^4.5, and 10^5.5 cm⁻³. The formatting is the same as in Fig. 3. The fits are shown as the solid black line (see legend). The modeled I_HCN/I_CO scales most directly (has a slope closest to unity) with f(n > 10^3.5 cm⁻³), supporting previous findings that this line ratio is sensitive to gas above moderate densities. Although not shown here, the same is found when comparing the emissivity ratio, ⟨ϵ_HCN⟩/⟨ϵ_CO⟩, with the same gas fractions. The Spearman rank coefficients are shown in the lower right corner of each plot.