function plot_dx4(x,y,dx,dy)

[row,col]=size(x); if row>1, x=x'; end
[row,col]=size(y); if row>1, y=y'; end

plot([x-dx;x+dx],[y;y],'k',[x-dx;x-dx],[y-dy;y+dy],'k',[x+dx;x+dx],[y-dy;y+dy],'k',x,y,'ok')