Galactic-scale properties of the sample. We plot against sin(|b|) E_B−V (top left), N(HI) (top right), N(H₂) = N(HCO⁺)/3 ×10⁻⁹ (bottom left), and W_CO, the line profile integral of the CO J=1–0 emission. The sight lines that were not detected in HCO⁺ absorption are shown as smaller green symbols in the upper two panels. The smaller green symbols in the lower panels are the 3σ upper limits on the plotted quantity. The solid line in each panel is a regression line: A plane-parallel vertically stratified Galactic gas layer would exhibit a power-law slope of –1. We also show averages at intervals of 0.01 in sin(|b|) in blue at the top. A legend marking the values of the Galactic latitude is inset near the top of the upper two panels. The values for the statistical parameters and the regression fits are given in Table 1.