Good question. For line graphs, we generally need the general extent to compute the graphs.
If we have three lines: that has its y dimension go anywhere from 0 to 500, we take the general extent to ensure all the data is visible.

We cannot use the same extents logic for stacked area (since they're offsets). One area stacks over the other. If we take the general extent what we get is the maximum of any layer in the stack and not that of all the stacked areas. What we actually need is to compute the extent by summing up all the data points for dimension for every corresponding point on the x dimension. To do that pretty easily through data selections, we need to transpose the matrix and sum the column and get the max out. It's done here: https://github.com/racker/glimpse.js/pull/163/files#L2R26