In general terms, the present invention relates to a self-propelled drawing device or a tracing device which can be for example an output peripheral for a desk top computer. The drawing device can be referred to as an "X-Y plotter". During the drawing, it should be able to access substantially all of the drawing medium. The drawing device draws a desired trajectory or locus or suitable approximation thereto as defined by computing or calculating means. The data to be plotted is held as a series of vectors in computer memory. Each vector is converted to the necessary movements required to reproduce the data on the drawing medium. The conversion calculations from vectors to movements may be carried out in the desk top computer, or in the drawing device or in a secondary device which interfaces between the two. The tracing device works in reverse in that it is made to follow a line by manual propulsion, and signal the trajectory or locus of the line or a suitable approximation thereto to computing or storage means. Both the drawing device and the tracing device act as a drafting peripheral in the form of a locus-tracking device in that the drawing device tracks a locus in order to draw a line and the tracing devices tracks a locus on a planar medium.