% this mfile plots an arrow with tail at the point (x0, y0) and
% tip at the point (x0+a, y0+b). Writing P = [x0, y0] and
% V = [a,b], the the complete call is arrow(P,V, color). The third argument,
% color, is optional. If the call is simply arrow(P,V), the
% arrow will be blue...