Fix the usage of `idxs` in plot recipes

Not all of the following work the way they should
```julia
plot(sol; idxs=(2,1))
plot(sol; idxs=[(0,1), (1,2)])
plot(sol; idxs=(0,1,2))
plot(sol; idxs=(1,1,2))
plot(sol; idxs=[(1,1,2),(0,1,2)])
```
We should make sure they all work.