function out=arrowhead(xin,yin,thetain,rmag,acolour);
%  function out=arrowhead(xin,yin,thetain,rmag,acolour);
%  x,y, theta must be 1D vectors, and of same dimensions
%  theta is the direction of the arrowhead
%  the arrowheads are of length rmag
%
x=xin;
y=yin;
theta=thetain;

[m,n]=size(xin);

if(m>n)
 x=x';
 y=y';
 theta=theta';
end 


phi=pi/8;

xtop=x+rmag*cos(-pi+theta-phi);
ytop=y+rmag*sin(-pi+theta-phi);
xbot=x+rmag*cos(-pi+theta+phi);
ybot=y+rmag*sin(-pi+theta+phi);
plot([x;xtop],[y;ytop],acolour);
hold on
plot([x;xbot],[y;ybot],acolour);