TreatNulls  
Line and Horiz. Line series can display "null" values in three different ways:

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.