Panel a: intensity contrast at the continuum of the Fe I 6173 Å line as a function of B from about disc centre (0.94 <  μ ≤ 1.00) to near the limb (0.10 <  μ ≤ 0.36). At each interval of μ, we binned the intensity contrast measurements by B in 40 G bins. The mean and standard deviation within each B bin is represented by the filled circles and error bars. As in Fig. 2, the standard error of the mean is negligible because of the size of our data set. The solid curves represent the bivariate polynomial in μ and B fit to measured intensity contrast (Eq. (2) and Table 1) and the dashed curves show the same from Yea13, which is almost indistinguishable because of the close agreement. Panel b: corresponding plots for the core of the Fe I 6173 Å line (red) and 1700 Å (blue). Again following Fig. 2, we scaled the Fe I 6173 Å line core values by a factor of 10.