套利合约之间的差值

下面是一个套利合约的差值计算。请问怎样实现两个两个套利合约之间的差值计算？比如塑料-PP的1月合约的差值减去5月合约的差值。如何使用spread？谢谢！

Params
 Numeric D0(1);
 Numeric D1(1);
 Numeric SW(0);
 Numeric Length1(5);
 Numeric Length2(10);
 Numeric Length3(20);
 Numeric Length4(30);
 Numeric Length5(60);
 Numeric Factor(100);

Vars
 Numeric OpenPrice;
 Numeric ClosePrice;

Begin
 If(Data1.Close == InvalidNumeric) Return;
 
 If(SW == 0)
 {
  OpenPrice = Data0.Open*D0 - Data1.Open*D1;
  ClosePrice = Data0.Close*D0 - Data1.Close*D1;  
 }Else If(SW == 1)
 {
  OpenPrice = Data0.Open/Data1.Open*Factor;
  ClosePrice = Data0.Close/Data1.Close*Factor;  
 }
 
 PlotNumeric("Open",OpenPrice);
 PlotNumeric("High",Max(ClosePrice,OpenPrice));
 PlotNumeric("Low",Min(ClosePrice,OpenPrice));
 PlotNumeric("Close",ClosePrice);
 PlotNumeric("MA1",AverageFC(ClosePrice,Length1));
 PlotNumeric("MA2",AverageFC(ClosePrice,Length2));
 PlotNumeric("MA3",AverageFC(ClosePrice,Length3));
 PlotNumeric("MA4",AverageFC(ClosePrice,Length4));
 PlotNumeric("MA5",AverageFC(ClosePrice,Length5));

End