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 was leaning toward single selection as default mostly for backwards compatibility with existing code.

However I still think it is up for discussion. Do you have any ideas for an heuristic to automatically decide between single and multiple, when no value is specified?