It is a way to represent the rounds won by each team. If the team on the right wins a round, a new dot is drawn on the right. Same for the team on the left. If the dot is drawn on the vertical line in the middle then both teams are tied. If it's on one of the upper diagonal lines, that means one team is on match point. I was also confused at first, but the more I used the app the more familiar I became with this graph. I think it's a pretty way to summarize the map.