Can't plot Vector of Quantity with open ended child type

This errors out when trying to `ustrip()` and `uconvert()`:

```julia
using Plots, Unitful

x = 0:10

function foo(x)
if(x > 5)
return 1u"A"
else
return 1.0u"A"
end
end

y = foo.(x)

plot(y)
```
`y` is type `Vector{Quantity{T, 𝐈, FreeUnits{(A,), 𝐈, nothing}} where T}`

Is this the intended behavior? It seems like for the purposes of plotting we should try to be as flexible as possible and accept as many inputs as possible.

Start by running the provided Plots and Unitful example and inspect the ustrip() and uconvert() paths that receive the Vector{Quantity{T, 𝐈, FreeUnits{(A,), 𝐈, nothing}} where T} value. Done means determining whether plotting should accept this open-ended child type and, if so, verifying that the example plots successfully.

Written by the indexing model from the issue text.