a graphic representation of the relation between the composition of a physicochemical system and the numerical values of its physical or mechanical properties (electrical conductivity, hardness, viscosity, index of refraction, and so on). A composition-properties diagram constructed for a constant temperature is called the isothermal curve of the property; for a variable temperature, the polythermal curve of the property; for constant pressure, the isobar of the property; and for variable pressure, the polybar of the property.