Click "edit graph" and change units to "natural log". Now look again. You'll see that the growth in margin loans is absolutely normal, and actually has only recently recovered from the dip caused by the 2008 financial crisis.
Great tip, but I didn't see "natural log" specifically. Perhaps "Compounded rate of change" is most applicable? That's still mostly above 0 historically, indicating margin usage is ever-increasing. The helpful graph would be margin usage as a weighted percentage of market participation.