HCN/CO versus Σ_mol relation. Here, we converted the CO(2–1) intensities into CO(1–0) intensities using the R₂₁ map introduced in Appendix A.1. Markers show 260 pc sightline measurements from NGC 4321, with dark blue markers denoting significant (S/N ≥ 3) data. The fit line was obtained by linear regression using LinMix to the stacked measurements (red hexagons) excluding data below ${\mathrm{\Sigma }}_{\text{mol}}=10{\mathrm{M}}_{\odot }{\mathrm{pc}}^{-2}$. The gold-shaded region shows the 1-sigma scatter of the detected sightlines about the fit line. In addition, we show best-fit relations from literature, covering kiloparsec-scale measurements of nearby galaxies (Gallagher et al. 2018a,b; Jiménez-Donaire et al. 2019; Neumann et al. 2023b), where Gallagher et al. (2018a) and Neumann et al. (2023b) use cloud-scale CO to measure cloud-scale Σ_mol and sub-parsec-scale galactic measurements of three nearby clouds (Tafalla et al. 2023).