The series hover interactivity uses the technique from lgrammel seen here: http://bl.ocks.org/1963983
It isn’t necessarily a tooltip, but data is displayed by inverting the x-axis value into a date, and mapping the date to the corresponding data value for the series.
If there is a simpler way to do this, I’d love to see an example, as I couldn’t find any other examples of this out there.
I think the vertical line adds a little bit of visual orientation to the streamgraph.
I left a few alternate color ranges in the code (which are essentially taken straight from colorbrewer), in case orange doesn’t sit well.