Global CO line ratios as a function of OH_max (left) and OH_EQW (right) for the sources with OH119 μm data as reported in the literature (see Table 1). The binned values (computed in the same way as in Sect. 5.4) are shown using darker colors and overplotted on the individual data points. Dashed lines indicate the best-fit relations obtained from a least squares regression analysis conducted for each transition line separately. The color coding refers to different line ratios, as indicated in the legend at the top-right corner of each plot.