Same as Figure 3, but now with contours of the observed quantities ﹩N({{\rm{H}}}_{2})﹩ and ﹩N({{\rm{H}}}_{3}^{+})/N({{\rm{H}}}_{2})﹩ in the plane of the model parameters ﹩{A}_{{\rm{V}}}(\mathrm{tot})﹩ and ﹩{\zeta }_{p}({\rm{H}})/{n}_{250}﹩. Blue contours are labeled with ﹩{\mathrm{log}}_{10}[N({{\rm{H}}}_{3}^{+})/N({{\rm{H}}}_{2})]﹩, and red contours are labeled with ﹩{\mathrm{log}}_{10}[N({{\rm{H}}}_{2})/{\mathrm{cm}}^{-2}]﹩.