This is why I wrote my own datapoint gathering script (I started it while at Techcrunch, curious about who scrolls down to comments, etc.).

It measures body focus, number of times unfocused, total focus time, scroll position, scroll rate and a bunch of other things.

It isn't completely working cross-browser yet, but it is close and useful enough where for Safari and Chrome visitors I can see how far down the page they are scrolling and how long the page is in focus for.