log N_H vs. log U_H phase-space plot, with constraints based on the measured total ionic column densities (sum of the column densities of the ground state and all the observed excited states). The measurements are shown as solid curves, and the dashed curves show lower limits. The shaded regions denote the errors associated with each N_ion. The phase-space solution with minimized χ² is shown as a black dot surrounded by a black ellipse indicating the 1σ error.