# multiple parametric_plot's with for loop

I'm studying differential geometry of curves and surfaces, and I have a question regarding multiple plots on one graphics, that is i want to plot multiple lines on one plot in for loop, but when I try to do that it shows only the last plot...

```
a,b,v,u = var('a b v u'); rectangular=(a,b,v);
z=0;
for i in range(81):
z=i*pi/20;z
pt=plot3d(z*v,(v,0,5),(a,0,5),(b,0,5),transformation=rectangular,plot_points=80,color="blue")
show(pt)
```