When I was surfing the FME Evangelist, a nice idea flashed in my mind. That is a practical usage of "Multiple Feature Attribute Support" functionality of the AttributeCreator in FME 2013 SP2. See this Q&A: Plotting xy in series. I didn't think there are more effective solutions than VariableSetter + VariableRetriever or Python script. But now, I know that only one AttributeCreator solve it elegantly.
I'll share this with you!
Calculate 'X' value:
Calculate 'Y' value:
Takashi