You can turn it on/off via “G” key on the keyboard. You can also use the Companion app to do the same. There is a button along the bottom w/the camera angle, U turn, toast, hammer, etc…, which will enable/disable the HR and power graph. Off the top of my head, I forget what it looks like. A graph maybe
It looks kinda like a graph and it’s the one after the camera. It’s not obvious what the button does initially since if you haven’t moved yet there’s no graph to toggle and you might have turned it off without realizing.