Savings Calculation - Compare Values ​​from different dates

[GustavoBotelhoTupy]

Hello Everybody!

I'm having some problems comparing prices of diferent years and using filters to show the savings in the current year. 

The Matematic equation is: Saving = (Price Paid - Average Price Last Year)*Quantity of this year

I used this equation for the Average price of last year:

Price2019 = Sum({$<Purchase_Date={">=01/01/2019 <=31/12/2019"}>}Purchase_value)/Sum({$<Purchase_Date={">=01/01/2019 <=31/12/2019"}>}Purchase_Quantity)

And this other equation to current year price:

Price2020 = Sum({$<Purchase_Date={">=01/01/2020}>}Purchase_value)/Sum({$<Purchase_Date={">=01/01/2020}>}Purchase_Quantity)

Then 

Saving = (Price2020 - Price2019)*Purchase_Quantity

But, when I apply the filter "Purchase_Date" to keep up with the current saving on the chart or table, an error occurs. 

The image below is without date filter

Saving Calculation - No Date selected

This other image has a date filter for 2020

Saving Calculation - 2020 Selected Date

Could anybody help me?