Overview of the relative difference in reduced equivalent width and (maximum) difference in normalised flux for five subsamples compared to the reference sample (N = 40). Each box plot contains information from all lines that satisfy the reduced equivalent width requirement shown in the legend. The bottom two panels show the relative difference in normalised line flux profile between the subsamples and the reference sample for two weak and two saturated fictitious Fe I lines with the largest and smallest maximum flux difference, respectively.