First thought was cool, clean simple API. I like it better than d3 to do the equivalent. -- Then I read this:

"When someone interacts with the graphics, for example by trying to drag an element to a new position, g9 optimizes over the space of possible values for your data to find a set of values that comes closest to creating the change. "

That is some highly magical DOM jimmying. High science weirdness. The applications I can think of where you'd need to manipulate a chart to modify data are all pretty niche (except for fun, which is universal). When the use case does come around this is going to be a delight.