Energy dashboard: Shows graph for gas but not for heat consumption

I have a sensor with state_class total_increasing for my heat consumption (warm water delivered from external) which works well. But adding it to the energy dashboard as an individual device only shows its total, and not a graph. Interestingly everything works perfectly well when I add the very same sensor for gas consumption. Why does it work as a gas consumption sensor (which it is not) and not as an individual device?