When I run the shortcode you provided, I get an axis of 39, which is correct for a stack chart. The stack formulae says to have both values fully represented one after the other (6, then 33). If I change 6 and 33 each to 13 and 13, I get two equally sized bars and the chart totals to 26.

This behavior is counter to what I think you want, which is an overlapped chart. For those, you have a bar at 6, then the fraction of 33 left (27), so the total axis is the size of the largest bar, not the summation of both.

Or maybe think of it as plotting two values: X, and 13 (where X is 0-13). The axis would be 13 no matter what value X takes on.

I do not yet support overlapped charts but think I can add it for you if it helps. Am I understanding your problem OK?