how to relate a measure to the date?

[acbi]

I have created a measure that tell me how much how much did the company spend on the last purchase of a product compared to the previus one. I would like to show the evolution of this data over the months, this is what a get when I try to do it:

 these are the formulas I used:

Saving =

[Valor médio do produto na penúltima compra]-[Valor médio do produto na última compra]

Valor médio do produto na penúltima compra =

CALCULATE(

[Valor médio produto],

FPEDIDODECOMPRA[Dt. Inclusao]=

MINX(

TOPN(2,FPEDIDODECOMPRA,FPEDIDODECOMPRA[Dt. Inclusao]),

FPEDIDODECOMPRA[Dt. Inclusao]))

Valor médio do produto na última compra =

CALCULATE(

[Valor médio produto],

FPEDIDODECOMPRA[Dt. Inclusao]=MAX(FPEDIDODECOMPRA[Dt. Inclusao]))

[amitchandak]

@acbi ,The information you have provided is not making the problem clear to me. Can you please explain with an example.
Can you share sample data and sample output in table format? Or a sample pbix after removing sensitive data.
Appreciate your Kudos.

[acbi]

Sure! 

We buy raw material that varies a lot in price, the ultimate goal is to show this variation over time graphically. This is an example of the table we currently use.

the information given to me for this are the product, the purchase date and the average price per unit purchased.