Plotting vector-valued functions when one component is constant

[jverzani]

See discussion here https://discourse.julialang.org/t/plotting-the-tangen-line-of-a-vector-valued-function/104297/2

@syms t
g(t) = [t+3/2*sin(2t),  4*cos(t), a*cos(2t)]
a = 1
plot(g(t)..., 0, 2pi) # okay
a = 0
plot(g(t)..., 0, 2pi) # error

[jverzani]

Closed by #527