Could you add a plot of cadence on the graph during a workout? This could be referenced to the right axis as HR and cadence have better overlap than power (and no need to complicate the UI with a third axis). There could be a setting to turn this off for those that don’t want that cluttering their power & HR curves.
One way to get a little better match between cadence and HR might be to set the top limit of the HR axis to be the ceiling modulo 10 of the entered max HR. So for me with HRmax = 165, the top of the graph would be 170, with a range of 50 - 170.