How to interpret heart rate plots on Apple watch

Apple WatchOS (and iOS Health) display heart rate data as vertical bars in a time series when viewing in the day interval. How do we interpret these bars? One might think the top and bottom of each bar represent max and min ranges, but then how to interpret dots? And what does it mean for there to be a gap in the bars? How do we determine the mean values in each interval? Below is an example of this kind of data, with the aforementioned bars, dots, and gaps.