1. A method of measuring an eye diagram, the method comprising the steps of: (a) repeatedly sampling the value of a periodic data signal at different times relative to a reference signal to create a stored data structure representing instances within the duration of this step that different (time, voltage) pairs are exhibited by the data signal when it is sampled; (b) traversing the data structure to construct in a frame buffer pixel information representing the eye diagram; (c) displaying the eye diagram represented by the frame buffer; (d) selecting an eye limits measurement; (e) positioning a measurement cursor icon to a selected location within the display of the eye diagram; (f) automatically extending horizontal and vertical lines from the measurement cursor icon until an examination of the data structure indicates that an end point on each extended line intersects the eye diagram; and (g) reporting the difference in the end points of the vertical lines as a voltage representing eye opening height and the difference in the end points of the horizontal lines as a time representing eye opening width.