It's histograms, according to the paper:
Fig. 5 Polar histograms from Fig. 4, resorted by descending φ from most to least grid-like (equivalent to least to greatest entropy)
Fig. 4 Polar histograms of 100 world cities’ street orientations, sorted alphabetically corresponding with Table 1
(A histogram shows how much there is of each kind, so here it's how much road there is per direction.)
Es ist eine Funktion, also funktioniert sie. Das liegt in ihrer Natur.