CO line ratios as a function of galactocentric radius, CO (2–1) brightness temperature, and TIR surface brightness. Each row shows the line ratio for individual lines of sight for fixed CO isotopologues but different transitions. Each column shows the CO line ratio as a function of galactocentric radius, CO (2–1) brightness temperature, and TIR surface brightness. The coloured data points show the individual lines of sight with a S/N > 5, and the small black downward triangles indicate 3σ upper limits. To illustrate the point density in the figure, we colour-code the points using a 2D KDE. Values are normalised to the peak density and range from most dense (1; white) to least dense (0; blue) The shaded area in each panel shows the 1σ (red) and 3σ (orange) censored regions for the individual lines of sight. They are an estimate of where we expect to no longer be able to detect line ratios (due to the low S/N of one of the lines). Marked in grey are the line ratios derived from the stacked spectra (downward triangles again mark upper limits). Error bars for significant points are indicated (for the stacked data points, the error bars are generally not visible due to the plotted point size being larger). For each line ratio, we used the highest resolution possible (indicated in the upper-left corner of the panel). To convert to Σ_mol, which we provide for comparison on the top x axis of each panel, we assume a constant R₂₁ = 0.89. We note that the x and y axes are correlated if the ¹²CO (2–1) emission is used in the line ratio. The stacked points allow us to probe the line ratios in fainter regions, which sometimes confirms the trend suggested by the individual (significantly detected) sight line measurements (e.g. the ¹²CO (2–1)/(1–0) in the top panel row) and sometimes reveals trends that could not be identified from the sight line measurements (e.g. the trend in the ¹²CO (3–2)/(2–1) ratio in the second row).