GP for empirical variance estimation. g(∆x) multiplies to capture the observational variances more correctly. Black squares with error bars are the logarithms of S², where S² is calculated as explained in the text. The solid black line is the mean prediction of g(∆x) from GP regression and the grey shaded are the lσ ranges. The y-axis label on the right shows S² and g(∆x) on a linear scale.