Hold on problem with 4 plots

[zemerlin]

hi this may seem a basic question...
i have a geometry project and i have to draw an elliptical cone and elliptical cylinder.
I want first to draw the generators then use meshgrid as if to cover my skeleton.
But my elliptical cone "skeleton" won't draw with the others, each time it removes previous plots. And if i put it in head of script then it blocks the rest...
This is the script i am using: thank you for your help.

axis([-20 20 -20 20 -40 40]);
axis equal;
hold on;

a=20; % focal axe half-lenght for cone
b=12; % non focal axe half-axe for cone
k=[-1:1/30:1];
h=[-20:20/30:20];
for v=0i/100:2*pi
z=20*ones(1,size(y,2));
x=a*cos(v);
y=b*sin(v);
plot3(x,y,z)
pause (10^(-20))
end
% draws the upper ellipse of the elliptical cone
for v=0i/100:2*pi
z=-20*ones(1,size(y,2));
x=a*cos(v);
y=b*sin(v);
plot3(x,y,z)
pause (10^(-20))
end
% draws the lower ellipse of the elliptical cone
a2=16; % focal axe half-lenght for cylinder
b2=10; % non focal axe half-lenght for cylinder
u=[-20:20/30:20];
for v=0i/30:2*pi
y=b2*sin(v);
z=a2*cos(v);
x=u;
plot3(x,y,z,'r')
pause(0.01)
end
% draws the generators of the ellipctical cylinder
for v=0i/30:2*pi
z=u;
x=a*k*cos(v);
y=b*k*sin(v);
plot3(x,y,z)
pause(0.05)
end
% draws the generators of the elliptical cone (here's my problem)

Thank you for your time