The effects of an irregularity of characteristic impedance in a transmission line are displayed graphically on a transmission-line circle diagram. This presentation shows qualitatively the major factors which determine the changes in voltage standing-wave ratio and phase produced by a single irregularity or by a large number of irregularities distributed along the line. This analysis is not limited to matched transmission lines as are the statistical methods. The resonance of a disc-spaced transmission line at super-high-frequency is considered. The method developed by Cotte for the determination of impedance variation along a transmission line by harmonic analysis of the distribution of irregularities is discussed in terms of the graphical approach, and a possible simplification to the experimental method is suggested.