The current value is compounded monthly. In other words, each month, the rate of return is applied to the balance so far, then the add monthly contribution is added. After 12 months, a point is added to the the plot for the year's ending balance.

To do this, we need a compounded monthly rate of return that yields the input annual return, which is not quite the annual return divided by 12 months:

    // monthly return is annual multiplier, to the 1/12th power
    monthlyReturn = Math.pow(1 + annRate/100, 1/12) 

    // grow previous value, then add monthly contribution
    value = prevValue x monthlyReturn + monthlyContrib

    inflationMult = infPerYear = (100 - inflation)/100
    for each year
        2003$ = value x inflationMult
        inflationMult = inflationMult x infPerYear

RMD is based on the Uniform Life Table published by the IRS. Essentially, at any given age, this table says how long one is expected to live yet. The balance in one's IRA is divided by that number, and the IRS requires you to take that amount as income for the year, and pay taxes on it. The IRS table and rules are quite likely to change if you're a long way from retirement, and you're supposed to use a different table if your spouse's age is more than 5 years different than yours, and other complications. But the essential nature of the rule is likely to remain: IRA savings are tax-protected until retirement, and then you're required to withdraw and pay taxes on the money, whether you want the money as income or not.

Because you have no control over RMD, your income may exceed the value entered for max annual income some years. That means that Social Security plus RMD came to more than the amount you requested for that year.

RMD is taken out of your savings balance once per year, in a lump sum at the end of the year. It is added to the Income value for the year.

    RMD = savings / lifeExpectancy (from a table)
    savings = savings - RMD
    income = income + RMD

This calculator assumes you want to live only on your earnings, not by withdrawing the bulk of your savings (until and unless you have to.) Given the guideline of max annual income, it tries to meet that by taking money from earnings, subtracting it from your balance, and transfering it to Income. If your RMD is high enough, the calculator may take nothing from earnings. If RMD + SS + earnings still doesn't reach your target income, the calculator doesn't draw down your savings, but just gives you a lower income.

The important point here is that Earnings is an available income stream, but is only tapped to try to meet your target income. Earnings is not added or subtracted directly from any other line.

Income is taken once per year, at the end of the year.

If the Income line is flat, in inflated dollars, that's a "fixed income." Because of inflation, a fixed income will be worth less each year.