docs: def calcTotal( self, values ):
          """
          Calculates the total for the given values.  For Datetime/Time objects,
          this will be seconds from minimum to maximum - defaulting to 1 second.
          For Date objects, this will be days from minimum to maximum, defaulting
          to 1 day.  For Number objects, this will be the sum of the list.  For
          Custom objects, a -1 value is returned.
          
          :param      values | [<variant>, ..]
          
          :return     <int>
          """
          rtype = self.rulerType()
          
          if ( not values ):
              return 0
          
          if ( rtype == XChartRuler.Type.Number ):
              return sum(values)
          
          elif ( rtype == XChartRuler.Type.Date ):
              values.sort()
              days = values[0].daysTo(values[-1])
              return max(1, days)
          
          elif ( rtype in (XChartRuler.Type.Datetime, XChartRuler.Type.Time) ):
              values.sort()
              secs = values[0].secsTo(values[-1])
              return max(1, secs)
          
          return -1