Grey points: residuals between the EW given by the polynomial GCOG and the EW of the library as a function of EW for 100 absorption lines (in the range 5212–5235 Å) at the grid point T_eff = 4200 K, log g = 1.4 and [M/H] = 0.0 dex. Black points: as before but for the line Fe i at 5231.395 Å alone. The residuals are normalized for the EW so that the values in the y-axis and the statistic in the panel express the errors of the polynomial GCOG normalized to the expected EW.