The orange points are actual skyline data, the green line is an approximation given by y(n) = A (1/n - 1/N) with A = 8500m and N = 104 is the total number of cities passing the threshold. Note that given N, changing the only free parameter A merely shifts the whole curve up or down on the log scale. This means that the fit is surprisingly good, despite the simplicity of y(n).
For a similar graph for individual buildings rather than city skylines, see the skyscraper heights analysis.