It'd be complicated in areas with flashbacks and whatnot. It would look pretty convoluted in the end. And really, the only real function it would serve is to orient people who aren't very familiar with the series, in which case, they shouldn't even be reading the Timeline.
It's something I've considered, especially with the kill roster. But ultimately, like I said, most people who are on the forums know the volumes in and out, so it'd just be superfluous.