In theory, as my fitness improves as the months pass, a specific pace (e.g., my easy pace) will seem… easier, which will be reflected in a lower HR at that pace if graphed over the weeks/months. Alternatively, for a given HR, I’ll be able to run faster, so at that HR, I should see my average pace increase over the weeks/months. It’d be great to be able to graph either pace (for a given HR) or HR (for a given pace) on the Y axis with days/weeks/months on the X axis to see fitness go up/down.