What does "stack" on Graph Builder actually do?

[#1]

I was playing around with stack on line on graph builder. I assumed it just did some sort of simple offset, but now looking at it, it actually calculates different values. Here are two plots of the same data using the "% of Factor" summary statistic, but one with stack, and the other without. The change is dramatic. What is stack actually doing?

[#2]

Here is the description of "Stack" taken from the Essential Graphing document found in the JMP Documentation Library

[#3]

So I had found the same documentation. But it doesn't answer why some plots look completely different after stacking, and I'm wondering if there are details about what stack actually does behind the scenes. It seems more than a simple offset.

[#4 · Accepted solution]

It is more obvious if you compare it to Area chart

Line no stack

Line stack

Area stacked

Area overlaid

And most likely the useful case for Line and stack is when you also use Fill