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.)