Returns the format that will be used for this ruler.
        
        :return     <str>

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