Main reason is because you're concatenating the two arrays [x x] and [y2 y3] as they are 1xN each so in the code you're just plotting like
X=[x x]; %which is now 1x2N
Y23 = [y2 y3] %which is also 1x2N
figure,plot(X,Y23)
to plot more you'll have to make put then as columns.
[ax,p1,p2] = plotyy(x,y1,[x',x'],[y2',y3'],'plot','plot');
the extra line is connecting the last point [x(end),y2(end)] and [x(1), y3(1)] together.
