Edge 830 Heart Rate Graph Display

Riding with my Edge 830 my HR graph is displayed, changing colour with each of the five HR zones. All fine there.

Can anyone explain the significance of the two black dots? I saw  post that said they were the rider and virtual partner and the two dots changed places. 

Can someone give me - and others I have spoken with - a clear explanation of what they actually are and how to read information from them?

I can find no information on the Garmin website.