A simple stacked coloumn graph built using d3's stack layout.
The SVG tooltip is based on this gist by Mike Bostock. In short, the tooltip's group, comprised of a rect and a text element, are created when the chart is first drawn and set to be hidden. Action is applied to the stacks' rects: on mouseover the group become visible; on mousemove the mouse's XY position is captured and used to transform/translate the group.
If you ajust the tooltip's XY offset, keep in mind that there can be no overlap between the group and the mouse's position or the group's visibility will remain hidden.