Why Isn't the Rolling HRV Chart Smooth?

It is understandable to expect a smoother line for your rolling HRV. On a trend of multiple readings, it would be. Within 1 single reading, it won't: 

Each point on the "HRV" line includes all points before it in the reading (will be smoother).

Each point on the "Rolling HRV" line includes only the points as far back as the set window, so it will be less smooth.