Seems like you just want a global variable that is shared between two cells?

    current_items = None

    def generate_numbers():
        global current_items
        items = []
        current_items = items
        while True:
            items.append(my_item)
        return items

    def view_current_plot():
        plot(current_items)

Note that you probably want to run `generate_numbers()` in a different thread or a different async coroutine so the Jupyter kernel would not block for the duration of it.