The blue area does not exactly represent deaths by preventable diseases. It is the area from the center to the edge of the blue. It is NOT a stacked chart, they regions are overlapping. You see this in November, when the black area is less than the red. She doesn't know how to represent this, so she simply makes the boundary line.

Overall, it was the first graph of its type, so for that it is interesting, but now that we know so much more, it isn't such a great example as some make it seem.