I think we conversed via email, however I'll also post my response here if any others are interested:
Unfortunately this isn’t something we currently support. However, I’ll certainly note down your interest as this is something we’re looking to add in the future.
For now, you may be able to work around this by creating a separate series which displays the penultimate and last datapoint and use the appendNumberOfDataPoints:toEndOfSeriesAtIndex: and removeNumberOfDataPoint:fromStartOfSeriesAtIndex: methods just on this extra series (i.e. you’ll need to remove the two datapoints and then append them with the updated data). This will cause the data to be reloaded for just this series, however I appreciate it’s not a very nice workaround.
Before trying the above however, it may be worth giving WizGod's suggestion a try!