CO 16–15/10–9 line ratio for all 19 sources for which both data sets are available. The spectra have been rebinned to 3 km s^-1 channels and only data points with S/N> 2 are included. The brightness of the color indicates the number of observations passing through a given point – brighter represents a higher number. Line ratios around ±2 km s^-1 are not included. Red is for Class 0 line wings, and blue for Class I. The mean ratio is displayed with a dashed line; if a linear scaling to correct for different beam sizes is applied the mean value shifts down to the dashed line marked “linear”, and similarly for a point-source scaling. The rotational temperature corresponding to a particular line ratio is shown on the second axis.