A visual graphical display of instantaneous mechanical vibration versus frequency over a selected frequency spectrum is presented to provide a vibration analyst with a prompt overall visual impression of the mechanical vibration characteristics of a rotating device which is under surveillance. In an alternative application, the visual display permits a rapid adjustment of an electrical bandpass filter of an associated vibration analyzer. The device employs several trains of diodes which are arranged so that one or more diode in each train is illuminated at any one moment.