Two-dimensional probability distribution functions of the fraction of mass of CH⁺ in the fiducial simulation d[M(CH⁺)/M_tot(CH⁺)]/[dlog(a) dlog(b)] displayed in a a = n_H and b = n_HT diagram (top panel), a a = x(CH⁺) and b = x(e⁻) diagram (middle panel), and a a = f(H₂) and diagram (bottom panel). The black solid curve in the top panel indicates the thermal equilibrium state expected for a gas illuminated by the Mathis interstellar radiation field and located at a visual extinction of 0.2 magnitude, which roughly corresponds to the visual extinction at the center of the standard simulation. The black dashed lines are isothermal contours at T = 60, 600, and 6000 K. The white contours in all panels show the isocontours that encompass 50% and 90% of the total mass of CH⁺, M_tot(CH⁺).