This adds a bit of debugging information on mouseover.
For example, a tooltip with the text `bird@tail.arm[0]` can be interpreted as "a bird symbol attached to the tail symbol's first arm attachment point."
The implementation is a bit funky: we basically annotate the SVG DOM with various `data` attributes, and on mouseover we traverse the DOM from the element the mouse is over all the way up to the SVG root element, picking out relevant `data` attributes and building a tooltip out of it. This ended up being easier than e.g. passing a bunch of props down the whole tree in React.
Atul Varma