If you have the vertices of the polygon, you can fit a striaght line and draw it.
Example:
P = [0 0 ; 1 0 ; 1 1 ; 0 1] ; % vertices of the polygon
p = polyfit(P(:,1),P(:,2),1) ; % fit a striaght line
xi = P(:,1) ;
yi = polyval(p,xi) ;
patch(P(:,1),P(:,2),'r')
hold on
plot(xi,yi,'b')
