How to create side by side CY and PY bar chart of aggregated measures?

Hi, i try to create a comparison for CY vs PY of the average of sales in a side by side bar chart.

where the period is handled by parameters.

i have made 4 calculations for this :

1.to count the average sales

2.to get the current year's value

3..to get the previous year's value

4.to get the latest year that was existed in the data source

i want that my chart only shows the current period (from Jan to selected month parameter) with the previous year's bar beside the current year's bar for the same month.

i have succeeded creating that chart if the measures is the sales, as shown below :

however, the PY data for the average sales are returned as NULL, as shown below :

any idea how to solve this issue?

really appreciate your help.

thanks

Ary