function y = PlaneFrameElementAxialDiagram(f, L)
%PlaneFrameElementAxialDiagram   This function plots the axial force 
%                                diagram for the plane frame element
%                                with nodal force vector f and length
%                                L.
x = [0 ; L];
z = [-f(1) ; f(4)];
hold on;
title('Axial Force Diagram');
plot(x,z);
y1 = [0 ; 0];
plot(x,y1,'k')