Way better transition animations

[Noleli]

It'd be cool if children came out of the filled in part of their parent's historgram.

Transitions are way harder than they could be because the width of the SVG element dynamically updates, and it's right aligned. That means all x values are updated every time, too. If the SVG element was made statically really wide and the scroll bar was toggled as needed, it'd be much simpler. Or just changing the width, recalculate all static object positions, then run the animation to add children.