Best-fit models for each individual cluster obtained by fitting the cross-calibration data of the MOS1/pn pair (left panel) or the MOS2/pn pair (right panel, repeated from the right panel of Fig. 3) using Method 2. 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 level. The dashed blue lines indicate the fourth-order approximation of the median. The MOS1/pn curve is repeated in the right panel as a dashed white-red line.