code: def stacked(requestContext, seriesLists, stackName='__DEFAULT__'):
      
      if 'totalStack' in requestContext:
          totalStack = requestContext['totalStack'].get(stackName, [])
      else:
          requestContext['totalStack'] = {}
          totalStack = []
      results = []
      for series in seriesLists:
          newValues = []
          for i in range(len(series)):
              if len(totalStack) <= i:
                  totalStack.append(0)
  
              if series[i] is not None:
                  totalStack[i] += series[i]
                  newValues.append(totalStack[i])
              else:
                  newValues.append(None)
  
          # Work-around for the case when legend is set
          if stackName == '__DEFAULT__':
              newName = "stacked(%s)" % series.name
          else:
              newName = series.name
  
          newSeries = TimeSeries(newName, series.start, series.end, series.step,
                                 newValues)
          newSeries.options['stacked'] = True
          newSeries.pathExpression = newName
          results.append(newSeries)
      requestContext['totalStack'][stackName] = totalStack
      return results
docs: Takes one metric or a wildcard seriesList and change them so they are
      stacked. This is a way of stacking just a couple of metrics without having
      to use the stacked area mode (that stacks everything). By means of this a
      mixed stacked and non stacked graph can be made
  
      It can also take an optional argument with a name of the stack, in case
      there is more than one, e.g. for input and output metrics.
  
      Example::
  
          &target=stacked(company.server.application01.ifconfig.TXPackets, 'tx')