This is a variation on the standard stacked area chart that uses a hover action to combat the shifting baseline problem.
Stacked charts are harder to read than others because the baseline of an area is shifted by the areas below it. A normal bar or area chart makes visual comparisons in different positions easy, because height measurements are made from the same starting place. This is not true when multiple areas are stacked on top of one another -- here, you have to adjust for the moving baseline yourself, which can be tricky.
This is especially true for stacked area charts, where the areas are continuous. The height of an area is always plotted vertically, but we perceive thickness perpendicular to its movement. When the area bends, we no longer see thickness as it is plotted. This is explained much more effectively by Dr. Drang, in a post that is also the source of the data for this gist.
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