Dynamic chart- curve dropping to zero ( chart type- Line)

Hi all i using following formula and description for the dynamic chart.
Please do the needful

In B1 = Project start date , C1= Mar-10
In B2= Project duration , C2= 30 ( which may vary)
In B3= Project Cost , C3= 10000000

In
C6=+IF(ROW()-ROW(C$6)+1=C$2,DATE(YEAR(C$1),MONTH(C$1)+ROW()-ROW(C$6),DAY(C$1)),"")
Till C50
In D5=IF(OR(B6="",B6=0),"",B6/$C$2) till d50
In
e5=IF(OR(D6="",D6=0),"",IF(D6=33.33%,(($I$1*D6^2) *100),IF(D6=66.67%,($I$2*D6+$I$3),IF(D6=66.67%,( $I$4+$I$5*D6+$I$6*D6^2)*100,"")))), till E50
In f5= IF(OR(B6="",B6=0),"",E6*$C$3), till F50
The problem i am facing is when i select the data for the chart ( C6:C50and
F5: F50) depending upon the value in C2 the curve will show but after that
curve drops to 0 , I don't want curve to drop 0 depend upon the value in C2