stacked bar charts

I want to set benchmarks in a stacked bar charts for each indivisual bar. I want to paint the bar whose Y value is below benchmark with one color and the portion between benchmark and the bar's Y Value with another color. Similarly the bars whose YValue exceeds benchmark value I want to paint the portion of the bar upto the benchmark with one color and another portion which exceeds the benchmark with different color. How can I achive this functionality?