nPer  script and chart function
This function returns the number of periods for an investment based on periodic, constant payments and a constant interest rate.
Syntax:
nPer(rate, pmt, pv [ ,fv [ , type ] ])
Return data type: numeric
Arguments:
Argument  Description 


The interest rate per period. 

The total number of payment periods in an annuity. 
pmt 
The payment made each period. It cannot change over the life of the annuity. A payment is stated as a negative number, for example, 20. 
pv 
The present value, or lumpsum amount, that a series of future payments is worth right now. If pv is omitted, it is assumed to be 0 (zero). 
fv 
The future value, or cash balance, you want to attain after the last payment is made. If fv is omitted, it is assumed to be 0. 
type 
Should be 0 if payments are due at the end of the period and 1 if payments are due at the beginning of the period. If type is omitted, it is assumed to be 0. 