I can only guess that the Histogram represents total score or something.
The symbols - no clue what they mean.
The fact that the barbarians appear to die out even if they don't - I suspect that it merely appears that they are gone but in fact the scale is too big to see their existence (very small line still there).
I think this is an area they streamlined too much - making it more difficult to understand. Why can't I choose to see the Histogram by score, by population, by gold, by culture, etc.?
There are some features that I just don't understand why they wouldn't be in the game - they don't detract anything. If you don't want to know that stuff, just don't look at the Histogram.