Line flux ratio diagrams for the sample galaxies. Ratios derived from Herschel-SPIRE data are marked by filled circles (single pointings) and filled triangles (map central peak), those derived from ground-based observations are denoted by open circles. Points corresponding to Centaurus A, Perseus A, and the Milky Way are indicated by an additional cross. a. (left) [C I] lines compared with adjacent CO lines. The linear regression correlation marked in the panel has a slope of 0.55, and a correlation coefficient r² = 0.66. b. (second from left) CO transition line ratios are likewise correlated with a slope of 0.17 and r² = 0.37. c. (second from right) [C I] line ratio versus the ratio of their neigbouring CO lines, slope 1.12 and r² = 0.67. d. (right) J = 2−1 CO isotopic ratio as a function of the CO (4−3)/CO (2−1) ratio; no obvious correlation although the dispersion increases with CO ratio.