I highly recommend changing this to `List(Instance(PlotObject, has_ref=True), has_ref=True)`, unless there is a case I'm not aware of (unlikely, because all new objects derive from `PlotObject`). As I said earlier, this isn't optimal solution, but it's enough here. Those `has_ref` attributes are annoying but I will be removing them in another round of improvements to properties (`has_ref` will be derived from property type).