Linear regression on UNION data

Hi IvoM ,

Please try using the REMOVEFILTERS function in the measure to remove the context filter.

TestForecastSixM-3+3TrendLine =
VAR _EndDate1 = EOMONTH(TODAY(), -1)
VAR _StartDate1 = EOMONTH(_EndDate1, -3) + 1
VAR _StartDate2 = EOMONTH(TODAY(), -1) + 1
VAR _EndDate2 = EOMONTH(_StartDate2, 2)
VAR CombinedData = 
    UNION(
        SUMMARIZECOLUMNS(
            DimCalendar[YYYY-MM],
            FILTER(
                ALL(DimCalendar[Date]),
                DimCalendar[Date] >= _StartDate1 && DimCalendar[Date] <= _EndDate1
            ),
            FILTER(
                ALL(DimPartProductFamily),
                DimPartProductFamily[FamilyType] = "Výroba"
            ),
            "Qty", SUM(FactsTransactionOOREC[Qty])
        ),
        SUMMARIZECOLUMNS(
            DimCalendar[YYYY-MM],
            FILTER(
                ALL(DimCalendar[Date]),
                DimCalendar[Date] >= _StartDate2 && DimCalendar[Date] <= _EndDate2
            ),
            FILTER(
                ALL(DimPartProductFamily),
                DimPartProductFamily[FamilyType] = "Výroba"
            ),
            "Qty", [ForecastSixMSumQtyNewestBackup]
        )
    )
VAR CombinedDataWithRank = 
    ADDCOLUMNS(
        CombinedData,
        "Rank", RANKX(CombinedData, [YYYY-MM], , ASC, DENSE)
    )
VAR AllCombinedDataWithRank = 
    CALCULATETABLE(
        CombinedDataWithRank,
        REMOVEFILTERS(DimCalendar),
        REMOVEFILTERS(DimPartProductFamily)
    )
VAR RegressionResult = 
    LINESTX(
        AllCombinedDataWithRank,
        [Qty], [Rank], TRUE
    )
VAR Intercept = MAXX(RegressionResult, [Intercept])
VAR Slope = MAXX(RegressionResult, [Slope1])
VAR CurrentRank = MAXX(CombinedDataWithRank, [Rank])
RETURN
    Intercept + Slope * CurrentRank

Please try it and let me know if it helps, and if it doesn't, please provide a Power BI Desktop file in progress (with sensitive information removed) that fully covers your issue or question in a usable format (not a screenshot). You can upload the PBIX file to a cloud storage service such as OneDrive, Google Drive (set up public access), SharePoint, or a Github repository, and then share the URL of the file.

If you have any other questions please feel free to contact me.

Best Regards,
Yang
Community Support Team

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If I misunderstand your needs or you still have problems on it, please feel free to let us know. Thanks a lot!