code: def offsetToZero(requestContext, seriesList):
      
      for series in seriesList:
          series.name = "offsetToZero(%s)" % (series.name)
          minimum = safeMin(series)
          for i, value in enumerate(series):
              if value is not None:
                  series[i] = value - minimum
      return seriesList
docs: Offsets a metric or wildcard seriesList by subtracting the minimum
      value in the series from each datapoint.
  
      Useful to compare different series where the values in each series
      may be higher or lower on average but you're only interested in the
      relative difference.
  
      An example use case is for comparing different round trip time
      results. When measuring RTT (like pinging a server), different
      devices may come back with consistently different results due to
      network latency which will be different depending on how many
      network hops between the probe and the device. To compare different
      devices in the same graph, the network latency to each has to be
      factored out of the results. This is a shortcut that takes the
      fastest response (lowest number in the series) and sets that to zero
      and then offsets all of the other datapoints in that series by that
      amount. This makes the assumption that the lowest response is the
      fastest the device can respond, of course the more datapoints that
      are in the series the more accurate this assumption is.
  
      Example::
  
          &target=offsetToZero(Server.instance01.responseTime)
          &target=offsetToZero(Server.instance*.responseTime)