Long-term history of Ni line energy for standard chips in 1-CCD mode (Figure 3a) and in 4-CCD mode (Figure 3b). Broken lines are apparent Ni line center energy and solid lines are that corrected for CTI and systematic energy shift in frame store region due to different pixel size. The dotted line is the expected K[[alpha]] line energy from neutral Ni (7.4723 keV).