There are two factors that effect the average stack size in BB given how many chips are in play:
- The number of players left in the tournament
- The size of the blinds
As the number of players left goes down, the average stack size in BB will increase if all else is kept the same.
As the size of the blinds increases, the average stack size in BB will decrease if all else is kept the same.
These two effects tend to balance each other out, but it largely depends on the type of tournament and how the players are playing. The overall trend will be a decrease in average stack size (BB) because the effect of blinds going up is much stronger than players getting knocked out on average stack size.
I don't think there is a mathematical graph or formula for this besides the basic formula for calculating the average stack size. The smaller the average stack size (in BB), players should be getting knocked out more frequently than if the average stack size is larger.
here is a rough visualization of how the graph would look, the graph goes upwards when players get knocked out and it takes a sharp decline when blinds increase. This would change a lot depending on how fast players are getting knocked out and how many are left in the tournament. the tournament would also end long before average stack size reaches 0BB.