function plotvector( v, s, sp )
% plot an arrow for the 2D-vector v
% v: the 2D vector
% s: a string passed to quiver for the color
% sp: if this string is set it defines the color of some helper lines

    if nargin < 2
        s = 'b';
    end
    quiver( [0.0], [0.0], [v(1)], [v(2)], 1.0, s );
    grid on;
    if nargin == 3
        hh = ishold;
        if ~hh
            hold on;
        end
        plot( [0.0 v(1) v(1)], [0.0 0.0 v(2)], sp );
        if ~hh
            hold off;
        end
    end
end