Point TreatNulls  
Three different ways to treat null values in FastLine series:

Series1.TreatNulls := tnDontPaint;
// <--- consider null points "empty"

Series1.TreatNulls := tnSkip;
// <--- jump lines to next non-null point

Series1.TreatNulls := tnIgnore;
// <--- consider null points as "zero"
// or any other "Y" value.