I think they must be stacked based on their description of amp as most of their traffic when it's the lowest line, but I agree the graph design is bewildering and does not read to me as stacked without thinking hard about what they are saying.
Yeah, I assume they are stacked. But they could do with better design to make that obvious (it isn't at all without inferring from the text) or some clarification in the text (or just with the graph key).