Bar charts less than zero

I'm trying to port a really nice chart I've seen drawn by hand on a wall into Mingle.  It's essentially a variant on the "stacked bar chart" concept.

The notion is to draw 2 stacked bars on the same date.  The twist is one bar goes up from zero, and the other goes down from zero.  In the example I'm using, the bar going up from zero is "defects added' and the bar going down is "defects closed." 

I find this is a nice and intuitive way to represent defects flows.  The center of the stacked rectangle tells us whether we had a net gain or loss of defects, and the size of the rectangle tells us how much defect activity we had.  This is superior to just graphing the net change in defects, because it shows the difference between "no new defects in or out" and "fixed 20 defects, but added 20 more."

Is this something the existing Mingle charting could handle?  I've tried hacking this in my changing my data: clause to be -1*Count(*) instead of Count(*), but MQL doesn't seem to like this (and I'm not sure it would give the chart I want anyways).