There's no units attached to those first examples so in effect there's nothing to render in the behavior over time graph. The system can still be rendered however.
In the later examples they start introducing numbers to the stock/flow nodes which then populates both graphs.
The "Behaviour over time" graphs don't seem to render for the hunger/chicken & egg/burnout/predator & prey examples.
There's no units attached to those first examples so in effect there's nothing to render in the behavior over time graph. The system can still be rendered however.
In the later examples they start introducing numbers to the stock/flow nodes which then populates both graphs.
Maybe that can be made clearer somehow