Wow!!!!, how is this thing even working? I don't understand what you have done . I feel dumb asking, but can you explain the settings a little more (although you have already showed all of them in the image above)
It just struck me that one could make a group of bars appear as one single bar when reducing the space in between to zero.
So I used a second calculated ValueLoop dimension to create clusters of three stacked bars each.
The side bars (1 and 3) here only get a value for the second expression, while the middle bar has values for both the first and second expression. The second expression then is reduced by the first expression value to let the middle stack height equal the side bar height in cases where the first expression is less than the second one.
I'm not convinced this explains it well enough ...?