Savings annuity calculator with graph help

Settings Remark
Time period (T) Time period (T) is the length of time that the money is in the savings account.
Decimal numbers can be entered.
The interest (I) is calculated at the end of each compounded period.

There are two ways to specify the time period:
• Method 1:
Enter a value and select the unit time period: days, weeks, bi-weeklies, semi-monthlies, months, bi-monthlies, quarters, semi-annuallies or years.

Using bi or semi in front of time periods means:
• bi = every two
For example:
bi-weekly = every two weeks
bi-monthly = every two months

• semi = twice a
For example:
semi-annually = twice a year
semi-monthly = twice a month

• Method 2:
Select a from date and to date.
The number of days between the from and to date is the time period.
The time period (T) equation:

 T = log((R / N) * ((FV / PMT) + (N / R))) and FV = N * T * PMT + I N * log(1 + (R / N))

where:
FV = Future value
I = Interest amount
R = Nominal interest rate per year (as a decimal, not in percentage)
T = Time period in years
N = Number of compounding periods in one year
PMT = Periodic payment amount, paid at the end of each payment period