Best-fit models for each individual cluster obtained by fitting the MOS1/pn cross-calibration data using Eq. (4) when applying Method 1 (left panel) or Method 2 (right panel). Each solid line connects the values of one cluster at the energy bin centres. The black symbols and vertical lines indicate the median and the standard deviation of the sample at each energy. The symbols from the right panel are repeated in the left panel in blue. The horizontal dashed line at unity indicates the expectation in the case of no cross-calibration bias.