Two-dimensional histograms of logarithmic proton-proton collisional age vs. solar wind parameters. Each histogram is normalized to its maximum value, and each histogram values is divided by the respective bin size before normalization. Each row corresponds to the same solar wind parameter (from top to bottom: proton speed v_p, proton density n_p, proton temperature T_p, magnetic field strength B, perpendicular variability of the magnetic field B_⊥/B, and oxygen charge-state ratio ). The first three columns refer to the three different Xu & Borovsky (2015) solar wind types (from left to right: sector-reversal plasma, helmet-streamer plasma, and coronal hole wind). The fourth column is the sum of the three other normalized histograms. In each panel, the sample size (N) is given as an inset.