In moving-medium XY plotters, a sheet of paper or other recording material is moved in an X-direction in accordance with one variable, such as time, and a pen is moved across the material in the orthogonal Y-direction in accordance with the other variable.  Often in these systems, the sheet of paper rises at certain points above the plane on which it lies.  Generally in these types of plotters there is no mechanism for estimating the vertical location of the paper if the paper does not contact the writing surface, or platen, on which it is to rest.  Thus, the pen may not be lifted sufficiently high by the plotter to clear the paper when the pen is moved to a new location in the raised position.  The result is that stray lines may be drawn from one location to another.  Raising the pen higher to solve this problem significantly increases the plotting time where many pen lifts are required, as in lettering for example. 
One solution to this problem is to put the paper over a roller while holding it taut with tension springs, vacuum, or the like, effectively eliminating any paper motion with respect to the roller surface.  The problem with such a system is that roller inertia is high, thereby limiting the speed with which the direction of motion of the paper can be changed.  Furthermore, engaging the tensioning mechanism is cumbersome and time consuming, and vacuum systems are relatively expensive.  Other plotters do not use this large roller concept, but instead draw the sheet of paper across the platen by means of sprockets which engage perforations along the paper's edge.  These plotters typically have paper buffers with slack loops on either side of the platen area to maintain the proper paper tension, and a motor driven wheel with which to play out or take up the paper roll as needed.  These systems substantially eliminate the problems of large roller inertia but they require control devices for the tensioning, and require a roll medium rather than individual sheets.