Plot directed

Thought for thermodynamic cycles, the function is used as the more common 2-D plot, but with one single arrow in the middle of the curve, that specifies the direction.
The length of the arrow can be chosen by user, as the opening angle and the LineSpecs.
% This function plots a 2-D line with the direction of the path.
% plot_d(X,Y,LineSpec,angle,length_line)
% - X and Y must be of the same length
% - angle determines the degrees of angulation of the arrow
% - length_line determines the lenght of the arrow (inverse direction if negative)
% Example:
% X=linspace(0,2*pi);
% Y=sin(X);
% plot_d(X,Y,'-b',45,0.3);