When we plot a process as a continuous line, curve, etc., of states using thermodynamic variables we are assuming that each point on the chart represents an equilibrium state, i.e., the process taking place is a series of “quasi-static” or “quasi-equilibrium” states. For example, refer to the T-S diagram below for a Rankine refrigeration cycle. The compressor, condenser, and evaporator processes are each reversible. The expansion valve (throttling) process is not. Note how it is represented by a series of hashed lines implying that the states between 3 and 4 are not equilibrium states. The enthalpy at state 3 and state 4 are the same but it is not a constant enthalpy process. Although Zemansky singles out the T-S diagram, the same holds true for other graphs of processes. The following is from an MIT web course on thermodynamics concerning processes.
“…properties define a state only when a system is in equilibrium. If a process involves finite, unbalanced forces, the system can pass through non-equilibrium states, which we cannot treat. An extremely useful idealization, however, is that only infinitesimal unbalanced forces exist, so that the process can be viewed as taking place in a series of quasi-equilibrium states”.
Hope this helps