function arrow(x1,y1,x2,y2)
% arrow(x1,y1,x2,y2)
%
% Places an arrow from (x1,y1) to (x2,y2) on an existing plot.

a=[pi/12,-pi/12];                   % Opening angles at tip of arrow.
s=15;                               % Length of arrow tip in pixels.

xa=get(gca,'xlim');
ya=get(gca,'ylim');
Lx=xa(2)-xa(1);                     % Axis lengths in plot units.
Ly=ya(2)-ya(1);
pxls=get(gcf,'position');           % Fig. dimensions in pixels.
pax=get(gca,'position');            % Axis dimensions.
Lxp=pxls(3)*pax(3);                 % Axis lengths in pixels.
Lyp=pxls(4)*pax(4);

line([x1,x2],[y1,y2],'color','k');  % Line part of arrow.
phi=atan2((y2-y1)*Lyp/Ly,(x2-x1)*Lxp/Lx); % Pointing angle of arrow.

dx=s*cos(phi+a)*Lx/Lxp;
dy=s*sin(phi+a)*Ly/Lyp;
line([x2,x2-dx(1)],[y2,y2-dy(1)],'color','k');
line([x2,x2-dx(2)],[y2,y2-dy(2)],'color','k');