While colour-highlighting can be a great way of emphasizing part or parts of a chart or diagram, it doesn’t usually add precise information, which was important to the client. To add this precise information we used a tooltip. But to make them as effective as possible we had to spend a bit of time refining their design.
Click through for several tips on tooltip design, including a technique to minimize tooltip occlusion (in other words, the tooltip blocking important information underneath it).