I have no clue either. “It just works”. I originally just copied the template of @masipila Control a water heater and ground source heat pump based on cheap hours of spot priced electricity - #152 by masipila I do not recall exactly how I got it working but I assume I just edited the yaml and added point names for the graphs. And poof,.they showed up.
Reading Marcus’ post he clearly states items have to be created for them to be available for the graph.