Note that the voltages of a voltage graph need not satisfy Kirchhoff's voltage law, that the sum of voltages around a closed path is 0 (the identity element of the group), although this law does hold for the derived graphs described below.
It's worth mentioning that the voltages in a voltage graph do not have to follow Kirchhoff's voltage law, which requires that the sum of voltages around a closed path be zero (the identity element of the group). However, this law does hold true for the more specific graphs described later on.