Median absolute correlation between [Fe/H] and R_birth for galaxies in the TNG50 Milky Way and Andromeda catalog across lookback time grouped by bar strength, with the shaded areas showing ±1 standard deviation about the median. The correlation between [Fe/H] and R_birth was calculated for age bins of 1 Gyr. To accommodate for outliers and higher feedback, the correlation was calculated for the running median, using R_birth bins of 0.25 kpc. We do not find that bar strength has an impact on the linearity of the average birth metallicity gradient. The large variability in the correlation between [Fe/H] and R_birth for stronger barred galaxies at smaller lookback times is predominantly due to weak metallicity gradients, even though the gradient itself is linear.