Unpivot Table LAMBDA Function

UNPIVOT.lambda

/*
Name: Unpivot Table (UNPIVOT)
Description: Given a table range with headers and array of header names, unpivots the 
   specified columns in place, optionally removing any blank entries.
Written By: Excel Robot (@ExcelRobot)
Category: Array
*/
UNPIVOT=LAMBDA(table,[columns_to_unpivot],[attribute_name],[value_name],[remove_blanks], LET(
   _ColumnsToUnpivot, IF(
      ISOMITTED(columns_to_unpivot),
      DROP(TAKE(table, 1), , 1),
      columns_to_unpivot
   ),
   _AttributeLabel, IF(ISOMITTED(attribute_name), "Attribute", attribute_name),
   _ValueLabel, IF(ISOMITTED(value_name), "Value", value_name),
   _FirstColumnToUnpivot, MATCH(INDEX(_ColumnsToUnpivot, , 1), INDEX(table, 1, ), 0),
   _UnpivotColumnCount, COLUMNS(_ColumnsToUnpivot),
   _ColumnNumbers, SEQUENCE(1, COLUMNS(table)),
   _IncludeColumns, (_ColumnNumbers >= _FirstColumnToUnpivot)
      * (_ColumnNumbers < _FirstColumnToUnpivot + _UnpivotColumnCount),
   _UnpivotColumns, FILTER(_ColumnNumbers, _IncludeColumns),
   _OtherColumns, FILTER(_ColumnNumbers, NOT(_IncludeColumns)),
   _FullOuterJoin, CROSSJOIN(
      CHOOSECOLS(table, _OtherColumns),
      VSTACK(_AttributeLabel, TRANSPOSE(_ColumnsToUnpivot)),
      TRUE
   ),
   _WithValues, HSTACK(
      _FullOuterJoin,
      VSTACK(_ValueLabel, TOCOL(DROP(CHOOSECOLS(table, _UnpivotColumns), 1)))
   ),
   _RemoveBlanks, IF(
      OR(ISOMITTED(remove_blanks), remove_blanks),
      FILTER(_WithValues, INDEX(_WithValues, , COLUMNS(_WithValues)) <> ""),
      IF(_WithValues = "", "", _WithValues)
   ),
   _ColumnOrder, LET(
      n, COLUMNS(_RemoveBlanks),
      s, SEQUENCE(1, n),
      IFS(
         s < _FirstColumnToUnpivot, s,
         s < _FirstColumnToUnpivot + 2, s + n - _FirstColumnToUnpivot - 1,
         TRUE, s - 2
      )
   ),
   _Result, CHOOSECOLS(_RemoveBlanks, _ColumnOrder),
   _Result
));

/*
Name: Cross Join Tables (CROSSJOIN)
Description: Given two tables, returns the Cartesian product of rows from the tables.
Written By: Excel Robot (@ExcelRobot)
Category: Array
*/
CROSSJOIN = LAMBDA(table1, table2, [has_headers], LET(
   _HasHeaders, IF(ISOMITTED(has_headers), TRUE, has_headers),
   _Data1, IF(_HasHeaders, DROP(table1,1), table1),
   _Data2, IF(_HasHeaders, DROP(table2,1), table2),
   _D1Rows, ROWS(_Data1),
   _D1Cols, COLUMNS(_Data1),
   _D2Rows, ROWS(_Data2),
   _D2Cols, COLUMNS(_Data2),
   _OuterJoinedData, MAKEARRAY(_D1Rows * _D2Rows, _D1Cols + _D2Cols,LAMBDA(i, j,
      IF(j <= _D1Cols, INDEX(_Data1, ROUNDUP(i / _D2Rows, 0), j), INDEX(_Data2, MOD(i - 1, _D2Rows) + 1, j - _D1Cols)))),
   _WithHeader, IF(_HasHeaders, VSTACK(HSTACK(TAKE(table1, 1), TAKE(table2, 1)), _OuterJoinedData), _OuterJoinedData),
   _WithHeader
));

Hey eric, how would you deal with unpivoting a multi-row header? We have this oftne in ecommerce offers, where mutliple size ranges are stacked in the header and the rows below show the quantities:
So something like this: