Would you consider adding a Pulse Pressure calculation to your reporting categories? It's the difference between Systolic and Diastolic pressure. It's not often considered, but it's a great measure of artery wall elasticity. Problem here is that you put Systolic on one chart, Diastolic on another. That's fine, as it would require a whole new UI to show them together.
Pulse Pressure, which is the calculation between the two, would show a measure of body health on the charts you have already. You'd just have to do the math, and have it appear as a Biometric Charts category.
I have been tracking this myself, as your incredibly well-thought app let me add a custom biometric then even charted it!!! The ask here is for you to do the math - so while Pulse Pressure wouldn't show up in the Diary it would appear in the Biometric Chart selection list.
This ask is a lot like the fiber ratios request - a calculated category in Biometric Charts.