Temperature and line-of-sight magnetic field variation over a slice of the atmosphere from where the four sample pixels were chosen. The pixel positions are given by vertical lines. The variation in δEW (panel a) and δV_max (panel b) across this slice are shown as blue and red curves. The former represents the case where only Fe is in NLTE, while in the latter Si, Al, and Mg are also treated in NLTE (see Sects. 5.3 and 5.4 for more details).