Hi all,

I might misunderstood the parametrized conversion to the code from this site:

I follow the parametrized helicoid and it becomes a cone:

```
using Plots;
gr()
function helicoid(r)
n = 100
θ = range(0, 2π; length = n)
u = range(0, 1; length = n)
x = r*u*cos.(θ)'
y = r*u*sin.(θ)'
z = θ*ones(size(u))'
return x, y, z
end
my_cg = cgrad([:red,:blue])
surface(helicoid(3), c=my_cg, label="helicoid", arrow=true, arrowsize=5)
```

another one, the `arrow=true`

does not showing any arrow. I try with helix that is working and the arrow is not there.

```
using Plots;
gr()
#plotlyjs()
# a helix is a curve in 3-dimensional space.
# The following parametrisation in Cartesian coordinates defines a particular helix;
function helix(r)
n = 100
θ = range(-10, 10; length = n)
x = r*cos.(θ)
y = r*sin.(θ)
z = θ
return x, y, z
end
my_cg = cgrad([:red,:blue])
plot(helix(5), c=my_cg, label="helix", arrow=true, arrowsize=5)
```