Calculate difference between Matrix Columns

Hi  EuanKonnektis , 

You coudld try to create an index column based on date group by name like below

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSlTSUTIyMDLQN9U3ATINlWJ1UATNQEx0QUNDINsYLJqEpt8IXRRkgKEBuijYBCOg4lgA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [name = _t, date = _t, amount = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"name", type text}, {"date", type date}, {"amount", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"name"}, {{"all", each _, type table [name=text, date=date, amount=number]}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Custom", each Table.AddIndexColumn([all],"index", 1,1)),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"all"}),
    #"Expanded Custom" = Table.ExpandTableColumn(#"Removed Columns", "Custom", {"date", "amount", "index"}, {"date", "amount", "index"})
in
    #"Expanded Custom"

 Then use measure like below

Measure 4 =
CALCULATE ( SUM ( t2[amount] ) )
    - CALCULATE (
        SUM ( t2[amount] ),
        FILTER ( ALLEXCEPT ( t2, t2[name] ), t2[index] = MIN ( t2[index] ) - 1 )
    )

Best Regards,
Zoe Zhi

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