Integrated emission line brightnesses for HCO⁺J = 1−0 (red boxes), CS J = 2−1 (green diamonds) and HCN J = 1−0 (magenta parallelogram) observed in diffuse gas, plotted against the integrated brightness of CO J = 1−0. Data sources are given in Sects. 4.1–4.3. Chained red and green curves are model results as discussed in Sect. 4.4. For both HCO⁺ (red) and CS (green), calculations are shown for clouds with N(H) = 0.7 and 2.0  × 10²¹  cm^-2 with number density n(H) increasing from 128  cm^-3 at left in marked steps of 2^1/2. X(HCO⁺) = X(CS)  = 2 × 10^-9 was assumed; the CS brightness increases linearly with increased abundance X(CS) at fixed W_CO while that of HCO⁺ declines very slightly, as discussed in Sect. 4.4 of the text.