tooltip to see exact numbers

[deanmalmgren]

One thing that is missing is the ability to quickly see what the specific numbers are on each bar in the visual. Perhaps we can incorporate this functionality only if d3.tip is included, just like how we handle expanded x-axis labels (#11).

I looked into this very briefly at one point and seem to recall that it may require to switching to using <rect> elements instead of <path> elements for all of the visuals. Could be a nightmare...