There's no direct way to do this in a native QV object. One option is to create another object that is a stacked bar chart behind the first object and have the formula of the stacked bar chart be dynamic. It this exact format is your requirement and the customer will not except an alternative this is something you can try.
I know you're looking for a technical solution and not advice on graph design, but here are my two cents' worth on a graph design that would give you similar results. If your graph had to be a line graph then coloring the area would be a little more helpful then just having the reference lines, but if your dimension is not time,dates or another continuous value then you should actually use a bar graph to compare distinct values. This is because the slop of the line doesn't give you much information when the dimension is made of distinct values. In which case, you could keep the reference lines and color the bars according to which area they are in and the user can then visually group the bars that are in the same group and the reference line will show them by how much they are in the group. The bar colors would be darker (more urgent) to lighter (less urgent) shades of the same color and not red, yellow and green since you should consider some people are color blind.
Regards.
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