Average price increase

[Simon4]

Hi all,

I am using the following formula to determen the % price increase of an item (difference between the price on the first order date and last order date). 

FirstSortedValue({<Netpurchaseprice={"<0>0"}>} Netpurchaseprice/Quantity,- Datecreated)/FirstSortedValue({<Netpurchaseprice={"<0>0"}>} Netpurchaseprice/Quantity,Datecreated)-1

It works when I select an item. On top of this, I want the average price increase per item per supplier. Right now, when I select a supplier it takes the first order date and last order date, but often these are totally different items. 

What I want: I want a KPI that shows average price increase. It has to work when i only select one item, a supplier or a itemgroup. I have the idea that I have to implement an aggr() with Itemcode. But at the moment I am not finding the right solution.

Thankyou in advance.