[Fe/H]_e vs. T_eff relation using the AP18 fine (top and second panels) and coarse (third panel) grids. The top panel shows the linear interpolation, and the two other panels show the cubic interpolation. The grey lines and symbols represent the solution (with an offset of −0.076) obtained with the GSL grid (cubic interpolation) and reported on Fig. 4. The other symbols and line styles are as in Fig. 4.