Illustration of how the median width of the star-forming region (and 25–75 percentile shaded region) in galaxies grows with cosmic time as a function of the present day bar strength using TNG50 MW/M31-like galaxies. Each line provides the fractional difference in the width of the star forming region, relative to a lookback time of 12 Gyr. In weaker-barred galaxies, where the width of the star-forming region increases by a factor of 10, the method of recovering the metallicity gradient from the range in [Fe/H] across age bins fails. When the width of the star-forming region does not change significantly over time (such as in the strongest barred galaxies), the method produces a smaller error. For the kilo-parsec values, we refer the reader to Figure C.3.