code: def _marker_line(self):
          # type: () -> str
          
          output = ''
          for col in sorted(self.col_widths):
              line = self.COLUMN_MARK + (self.DASH * (self.col_widths[col] + self.PADDING * 2))
              output += line
          output += self.COLUMN_MARK + '\n'
  
          return output
docs: Generate a correctly sized marker line.
  
          e.g.
  
          '+------------------+---------+----------+---------+'
  
          :return: str