Multiple selection

This is a first implementation of multiple selection that kind of works on the WorldBank data viz.

So far by default the time variable is single selection and all other variables are multiple selection. That needs to be configurable at compile time.

Some multiple selection updates are not instantaneaous -- why?

Also the lines sometimes update and sometimes do not -- why?

http://bl.ocks.org/tdhock/raw/b1a59324286448047961/
I'm curious why you choose single selection for the default. What potential problems do you see for having multiple selection by default?