This result is illustrated graphically in Fig. 2.7, which makes use of the fact that an
integral can be interpreted as an area. The magnitude of w, denoted |w|, is equal to the
area beneath the horizontal line at p = pex lying between the initial and final volumes.
A p,V-graph used to illustrate expansion work is called an indicator diagram; James
Watt first used one to indicate aspects of the operation of his steam engine.