Power Query/DAX to calculate monthly raw sales figure - powerbi

Question

Dear stackoverflow, please help!
I'm hoping for some assistance with data processing in Power BI, either using Power Query or DAX. At this point I am really stuck and can't figure out how to solve this problem.
The below table is a list of sales by Product, Month, and Year. The problem with my data is that the value in the sales data is actually cumulative, rather than the raw figure of sales for that month. In other words, the figure is the sum of the number of sales for the month (for that Year and Product combination) and the number of sales for the preceding month. As you will see in the table below, the number gets progressively larger in each category as the year progresses. The true number of sales for TVs in Feb of 2021, for example, is the sales figure of 3 minus the corresponding figure for sales of TVs in Jan of 2021 (1).
I really would appreciate if anyone knows of a solution to this problem. In reality, my table has hundreds of thousands of rows, so I cannot do the calculations manually.
Is there a way to use Power Query or DAX to create a calculated column with the Raw Sales figure for each month? Something that would check if Product and Year are equal, then subtract the Jan figure from the Feb figure and so on?
Any help will be very much appreciated,
Sales Table
Product
Sales (YTD)
Month
Year
TV
1
Jan
2021
Radio
4
Jan
2021
Cooker
5
Jan
2021
TV
3
Feb
2021
Radio
5
Feb
2021
Cooker
6
Feb
2021
TV
3
Mar
2021
Radio
6
Mar
2021
Cooker
8
Mar
2021
TV
5
Apr
2021
Radio
7
Apr
2021
Cooker
8
Apr
2021
TV
7
May
2021
Radio
8
May
2021
Cooker
8
May
2021
TV
9
Jun
2021
Radio
10
Jun
2021
Cooker
10
Jun
2021
TV
10
Jul
2021
Radio
10
Jul
2021
Cooker
10
Jul
2021
TV
11
Aug
2021
Radio
13
Aug
2021
Cooker
12
Aug
2021
TV
11
Sep
2021
Radio
13
Sep
2021
Cooker
12
Sep
2021
TV
12
Oct
2021
Radio
14
Oct
2021
Cooker
13
Oct
2021
TV
17
Nov
2021
Radio
19
Nov
2021
Cooker
17
Nov
2021
TV
19
Dec
2021
Radio
20
Dec
2021
Cooker
20
Dec
2021
TV
4
Jan
2022
Radio
2
Jan
2022
Cooker
3
Jan
2022
TV
5
Feb
2022
Radio
3
Feb
2022
Cooker
5
Feb
2022
Thanks, Jim

Answer 1

Give this a try in powerquery / M. It groups on Product and Year, then sorts the months, and subtracts each row from the next row to determine the period amount.
let Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
#"Grouped Rows" = Table.Group(Source, {"Product", "Year"}, {
{"data", each
let r=Table.Sort(Table.AddIndexColumn(_, "Index", 0, 1),{ each List.PositionOf({"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"}, [Month]), {"Month",Order.Ascending}}),
x= Table.AddColumn( r, "Period Sales", each if [Index]=0 then [#"Sales (YTD)"] else [#"Sales (YTD)"]-r{[Index]-1}[#"Sales (YTD)"])
in x
, type table }
}),
#"Expanded data" = Table.ExpandTableColumn(#"Grouped Rows", "data", {"Sales (YTD)", "Month", "Period Sales"}, {"Sales (YTD)", "Month", "Period Sales"})
in #"Expanded data"