def defined_name
      return unless range
      Axlsx.cell_range(range.split(':').collect { |name| worksheet.name_to_cell(name)})
    end
the formula for the defined name required for this auto filter
This prepends the worksheet name to the absolute cell reference
e.g. A1:B2 -> 'Sheet1'!$A$1:$B$2
@return [String]