These substances are defined in another plot of chemical potentials: Chemical_potentials_vs_log_mole_fraction.svg. Unfortunately this figure is tiny and I don't think I can fit the labels.

Some features of note:

• Miscible liquids are usually defined to have activity coefficient = 1 for pure state.
• Other solutes are defined to have activity coefficient = 1 for infinitely dilute state. The curves stop at some point because they reach saturation.
• For electrolyte (purple curve) it's the mean activity coefficient. It slowly approaches 1 due to ionic effects (Debye-Huckel limiting form) -- this is the ${\displaystyle -\sqrt{x}}$ form that one usually sees in plots where ${\displaystyle x}$ axis has linear scale.