Linear regression in Power Query with grouping

Here's an example similar to yours that demonstrates what I was suggesting:

let
    Forecast = (sourceTable as table, xcol as text, ycol as text) =>
    let
        range = Table.SelectRows(sourceTable, each Record.Field(_, ycol) <> 0),
        rowcount = Table.RowCount(range),
        sumx  = List.Sum(Table.Column(range, xcol)),
        sumx2 = List.Sum(List.Transform(Table.Column(range, xcol), each Number.Power(_, 2))),
        sumy  = List.Sum(Table.Column(range, ycol)),
        sumxy = List.Sum(Table.TransformRows(range, each Record.Field(_, xcol) * Record.Field(_, ycol))),
        avgx  = List.Average(Table.Column(range, xcol)),
        avgy  = List.Average(Table.Column(range, ycol)),
        Slope = ((rowcount * sumxy - sumx * sumy) / (rowcount * sumx2 - Number.Power(sumx, 2))),
        Intercept = avgy - Slope * avgx,
        Result = Table.AddColumn(sourceTable, Text.Combine({ycol, "Forecast"}), each
            if Record.Field(_, ycol) = 0
            then Intercept + Slope * Record.Field(_, xcol)
            else 0, type number )
    in
        Result,
        
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("ZZBRCsAgDEPv0m8/pkbHziJ+bPc/xIw4XOhHCTxC0rY1ixbsHkNNxXrYKHGioEznKQhjDiHFkSrkWXUoglgHdbEuqwsuatel3y1VCDWqaZ4CQXBJmv0tfimiVkVzcUVwUT58/am/", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [dimA = _t, dimB = _t, dimX = _t, FactY = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"dimA", Int64.Type}, {"dimB", type text}, {"dimX", Int64.Type}, {"FactY", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"dimA", "dimB"}, {{"SubTable", each Forecast(_, "dimX", "FactY"), type table [dimA=nullable number, dimB=nullable text, dimX=nullable number, FactY=nullable number, FactYForecast=nullable number]}}),
    #"Expanded SubTable" = Table.ExpandTableColumn(#"Grouped Rows", "SubTable", {"dimX", "FactY", "FactYForecast"}, {"dimX", "FactY", "FactYForecast"})
in
    #"Expanded SubTable"

The highlighted column is the result: