One of the most interesting graphic functions is the diagram function. The term diagram is used to describe the process of plotting a graph which represents the relationship between two recorded parameters of flight data. The system allows the presentation of up to four diagrams per screen for three screens (i.e. twelve diagrams in all). The diagrams are helpful for engine specialists. Such a function is used to test the quality of the engine. It has been proved that, when the on screen graphics representing the correlation between the vibration and rotation begins to take up a large part of the enclosure, the engine is out of order or inefficient.