Heart beat graph on Apple Watch Series 8

When the lines are broken, that means that your heartbeat has jumped to a different HR. This doesn’t show Atrial fibrillation but you should always see a doctor for questions like this.

I don’t know what the broken lines mean, but if you have aFib, your watch8 will detect it if you tell it to scan for it. More, if detected, your watch will prompt you to see your doctor who will perform their own aFib tests. They have the power to formally diagnose you, or not.