Monthly Inventory

Hello Community,

I am new to power BI and Im having some isssues finding the best practice to use the dax functions in order to calculate the monthy inventory. Currently I only have the current existing inventory of each day(this report doesnt have any dates since it only shows the amount of stock I currently have), the sales and purchase order. 

Can someone help me find a way to create the formulas so I can see the inventory level in the previous months.

For example, In the dashboard below, my aim is to type a item num and see the inventory levels from the previous months in the chart. Currently its shown as separeted values.

Hopefully I managed to explain myself, in case there is something that needs to be clarified please let me know.

Thanks in advance...

Yuss I.