docs: def format( self ):
          """
          Returns the format that will be used for this ruler.
          
          :return     <str>
          """
          if ( self._format is not None ):
              return self._format
          
          rtype = self.rulerType()
          if ( rtype == XChartRuler.Type.Number ):
              self._format = '%i'
          
          elif ( rtype == XChartRuler.Type.Date ):
              self._format = 'M.d.yy'
          
          elif ( rtype == XChartRuler.Type.Datetime ):
              self._format = 'M.d.yy @ h:mm ap'
          
          elif ( rtype == XChartRuler.Type.Time ):
              self._format = 'h:mm ap'
          
          return self._format