Dynamic placement of category baselines

[markplace]

We've got a category chart with 2 nested categories on the x-axis, kinda like this:

100%|                 *                                             *
         |  *                                                   *                *
50%  |
         |          *            *                      *
0%    |                                 *     *
          ------------------------------------------------------------------
         | Q1 | Q2 | Q3 | Q4 | Q5 | Q6 | Q7 | Q8 | Q9 | Q10 |
         |     Low   |       Med        |                High                 |

We need to show category baselines separating each of the secondary categories so that we have vertical lines delineating Low, Med, and High.

If I define two category baselines with hard-coded layout calculations (e.g. 2.5 and 5.5) then everything looks great.  The problems is that the number of questions per category is dynamic. 

Can anyone provide insight on how to dynamically specify the baseline locations?

Thanks!