What is important to recognize from those graphs are trends
Also it is good to compare what was happening in UFC and overall MMA.
Here is one analysis –
Comparing HW UFC division with the overall HW for period 2006 - 2012
check the dark brown line in both graphs
In 2006 UFC HW division was in a very poor shape (check the beginning of the dark brown line).
Most of the best HW where in Pride.
The best that UFC had to offer was Tim Sylvia and Andrei Arlovski (ranked #8 and #9 in the world).
They also had Brandon Vera (#11) and Jeff Monson (#13). And that is it for top 20.
Then Pride died.
Some of the Pride HW went to UFC (Big Nog, Werdum, Cro Cop).
Randy Couture returned to HW in a big way (obliterating Sylvia).
In the last quarter of 2007 UFC HW division peaked.
From the top 15 HW, they had 9.
They didn’t have Fedor (#1) and Barnett, but everybody else was there.
Then in 2008 Affliction created very strong HW division (Fedor, Barnett, Arlovski, Rothwell, Tim Sylvia, Pedro Rizzo)
and that weakened the UFC strength at HW.
Remember, Couture left the UFC to fight Fedor,
Affliction died. But most of the HW fighters went to Strikeforce, not UFC.
That is finally where UFC decided that all the best HW need to be under Zuffa.
Once that Strikeforce tournament was over and Fedor was dethroned, there was no more competition.
In 2012, UFC had all the best HW in the world.
At the end of overall graph, you can see that dark brown line is going down. That means overall HW division is dropping in popularity and fighters are moving down. Except UFC, no other HW title means anything.
That is not the case with the UFC HW. It is a pretty stable line. That is because UFC sucked in all the best HW.