1. A curve generator for a display system, the curve generator comprising arc generation means for generating an arc of a circle from data defining the locations of two end points and an intermediate point on the arc, wherein the arc generation means comprises initialization means for calculating the angle subtended between a first vector, from a first of the end points to the intermediate point, and a second vector, from the second of the end points to the intermediate point, and arc plotting means responsive to said initialization means for defining a succession of further vectors from said first end point and for calculating, for each further vectors, a point of intersection with a counterpart vector, from said second end point, with which said angle is subtended, whereby a succession of further points are plotted on the circular arc.