[x,y]=meshgrid(0:0.2:5, -3:0.2:3);
dy=x-y;          %y'=x-y
dx = ones(size(dy));
dyu = dy./sqrt(dx.^2+dy.^2);
dxu = dx./sqrt(dx.^2+dy.^2);
quiver(x,y,dxu,dyu, 'MaxHeadSize',0)
axis square
grid on
axis([0 5 -3 3])