ZigZag for Amibroker (AFL)

//z_ZigZagValid
// ******** CHARTING 
PercentChange = 6;
mystartbar = SelectedValue(BarIndex());  // FOR GRAPHING

mystartbardate = LastValue(ValueWhen(mystartbar == BarIndex(), DateNum(),1));

InitialValue = LastValue(ValueWhen(mystartbardate == DateNum(), C , 1 ) ) ;
Temp1 = IIf(BarIndex() >= mystartbar, InitialValue, Null) ;
Plot(Temp1, " ", colorBlack,styleLine);
Plot((1+(LastValue(PercentChange)/100))*(Temp1), " ", colorGreen, styleLine) ;
Plot((1-(LastValue(PercentChange)/100))*(Temp1), " ", colorRed, styleLine) ;

ZZ = Zig(C,LastValue(PercentChange)) ; 
PivotLow = Ref(IIf(Ref(ROC(ZZ,1),-1) < 0 AND ROC(ZZ,1) > 0, 1, Null),1);
PivotHigh = Ref(IIf(Ref(ROC(ZZ,1),-1) > 0 AND ROC(ZZ,1) < 0, 1, Null),1);

PlotShapes( shapeCircle*PivotLow, colorGreen,0, L, -20) ; 
PlotShapes( shapeCircle*PivotHigh,colorRed,0,H, 20) ;

Buy_Valid = IIf(C>(1+(LastValue(PercentChange)/100))*(ValueWhen(PivotLow, C,
1))
AND ROC(ZZ,1) > 0,1,0); 
Sell_Valid = IIf(C<(1-(LastValue(PercentChange)/100))*(ValueWhen(PivotHigh, C,
1))
AND ROC(ZZ,1) < 0,1,0); 

Buy_Valid = ExRem(Buy_Valid,Sell_Valid);
Sell_Valid = ExRem(Sell_Valid,Buy_Valid);

PlotShapes( shapeUpArrow*Buy_Valid, colorGreen,0, L, -20); 
PlotShapes( shapeDownArrow*Sell_Valid, colorRed,0,H, -20) ;