PQ - Running Total start over

Running Total start over

Date	Expense		Date	Expense	RTSO
01/01/2020	1		01/01/2020	1	1
02/01/2020	2		02/01/2020	2	3
03/01/2020	3		03/01/2020	3	6
04/01/2020	4		04/01/2020	4	10
05/01/2020	5		05/01/2020	5	15
06/01/2020	6		06/01/2020	6	21
31/01/2020	7		31/01/2020	7	28
01/02/2020	8		01/02/2020	8	8
02/02/2020	9		02/02/2020	9	17
05/02/2020	10		05/02/2020	10	27
02/03/2020	11		02/03/2020	11	11
03/03/2020	1		03/03/2020	1	12

Code:

---

// Table1
let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    Month = Table.Sort(Table.AddColumn(Source, "Month", each Date.Month([Date]), Int64.Type),{{"Month", Order.Ascending}}),
    TableType = Value.Type(Table.AddColumn(Source, "RTSO", each null, type number)),
    Group = Table.Group(Month, {"Month"}, {{"AllData", fnRTSO, TableType}}),
    Expand = Table.ExpandTableColumn(Group, "AllData", {"Date", "Expense", "RTSO"}, {"Date", "Expense", "RTSO"}),
    TypeDate = Table.TransformColumnTypes(Expand,{{"Date", type date}}),
    TSC = Table.SelectColumns(TypeDate,{"Date", "Expense", "RTSO"})
in
    TSC

// fnRTSO
(MyTable as table) as table =>
let
    Source = Table.Buffer(MyTable),
    TableType = Value.Type(Table.AddColumn(Source, "RTSO", each null, type number)),
    Cumulative = List.Skip(List.Accumulate(Source[Expense],{0},(Cumulative, Value) => Cumulative & {List.Last(Cumulative) + Value})),
    RTSO = Table.FromColumns(Table.ToColumns(Source)&{Cumulative},TableType)
in
    RTSO

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