Correlation between line centroid and integrated line signal intensities (in log scale) in different areas of the CCDs. Each CCD was divided into four stripes that are parallel to the cross-dispersion direction, and each stripe covers an interval of 1024 pixels along the main dispersion direction. All LFC lines from the 160 spectra were used to produce this plot. Lines are grouped into 100 flux bins in each subplot (stripe). Exponential functions are used to fit the line shifts as a function of the measured line flux. The fit result is shown with the dashed red line. The relative drift Δμ is measured in pixel space, compared with the reference acquisition, i.e. the acquisition with the highest averaged counts (or lowest attenuation).