Power BI Report Builder - Use first row as header

Hi belljbx ，

You could  pivot data by PowerBI Desktop ,then  create a new dataset in report builder and select enter data as the type, and copy table visual data from powerbi desktop to report builder's dataset.

base table:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45Wci5KTSxJTVFwAZJKOkpGBkaG+pb6RhZANgjF6kQr+aQmFqWkFilk5imUFCVm5mXmpYNUmgIJQwMQYaoUGwsA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Month = _t, #"2021/9/30" = _t, #"2021/10/31" = _t, Column1 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Month", type text}, {"2021/9/30", type text}, {"2021/10/31", Int64.Type}, {"Column1", Int64.Type}}),
    #"Unpivoted Columns" = Table.UnpivotOtherColumns(#"Changed Type", {"Month"}, "Attribute", "Value"),
    #"Renamed Columns" = Table.RenameColumns(#"Unpivoted Columns",{{"Attribute", "Month2"}}),
    #"Pivoted Column" = Table.Pivot(#"Renamed Columns", List.Distinct(#"Renamed Columns"[Month]), "Month", "Value")
in
    #"Pivoted Column"

Final:

Then copy:

And about pivot ,you also could refer similar case:

https://community.powerbi.com/t5/Desktop/Pivot-multiple-columns/m-p/223138 

And  create a new dataset in report builder :

For details, you can refer to the steps given in the official documentation.

Enter data directly in a paginated report in Report Builder - Power BI | Microsoft Docs 

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Best Regards

Lucien