Quantum interference (QI) effect in molecular devices introduces an abrupt dip in the transmission spectra, thus it can be used to a series of applications such as molecular switches and molecular thermoelectric devices. For the development of the QI-based molecular devices, it is important to give the guideline to predict and to control the shape and positions of QI in transmission spectra. For this purpose, we present a simple graphical approach (called as "parabolic model") to predict the appearance, shape, and energy position of the QI in transmission spectra for T-shaped molecular devices. We show that this "parabolic model" enables one to visualize the relationship between key electronic parameters and the shape of transmission functions without calculating transmission functions.