Polar graphs for differential cross section (dcs) called spatial dcs maps are presented as graphical representation of the angular distribution of vibrationally inelastic electron scattering by polyatomic molecules. The objective of this paper is to show that an intuitive understanding of the principal features of these graphs can be obtained from a simple analysis of the normal modes of vibration of the target molecule and plane-wave functions representing the scattering electron. The procedure is illustrated on the H2 and CH4 molecules.