Dimension limit for middle result

[#1]

Hello,

I have a classic bar chart showing Product Category in axis X and sum(SELLING_PRICE) in axis Y ordered by Y-Value descending.

I need to divide this chart in 3: Top Performing, Mid Performing, Bottom Performing depending by Y-Value.

My unique selection is Period because in each period I don't sell same PRODUCT_CATEGORY and I need to show only PRODUCT_CATEGORY sold in selected Period.

My Idea is making 3 charts , the first with Top N , the 3th with Bottom M , the 2nd with Medium P Product Category performer.

I create a Variable

     vTotal = Count( distinct {<PERIOD={$(vCurrentPeriod)}>} PRODUCT_CATEGORY)

, calculated N  for 1st chart:

     vN = Floor(vTotal /3)

, calculated P

     vP = Floor(vTotal /3)

, calculated M

     vM = vTotal - vN - vP

So, for first chart I used Dimension Limit for TOP vN elements, for 3th chart I used BOTTOM vM elements... and for 2nd chart?

How can I select elements in the middle?

Could I define a PRODUCT_CATEGORY set including all PRODUCT_CATEGORY not in 1st chart and not in 3th chart?

Someone has few suggestion for do this?

I'm using QlikView 11.20