I don't really see that anything can be inferred from this chart. Markets 101: markets move up when there are more buyers than sellers, and down when more sellers than buyers.
The market has steadily been going up, so net there have been more buyers (could be one institution or person continuing to buy also, doesn't matter). Currently buying stocks is much more expensive than it was 10 years ago, so it makes sense that one would need more buying power today to hold the same amount of stock, which would increase your margin requirements. However at the same time your position (assuming you didn't sell) has also increased in value. So if there was a large correction you're not going to be margin called unless you have insufficient capital to hold the position, which if you bought a long time ago has to be a huge drop.
Basically all this chart tells me is that as prices go up more investors are holding stock (because more bought), and as prices go down investors have more capital available (because they sold).