definition

A straight-line projection is a calculation that uses an assumed annual total return for a portfolio, and applies it equally for each year of the projection.   The resulting future value of the portfolio based upon that level assumption is illustrated.

commentary

In the real world,  returns are not static;  they fluctuate.  A straight-line projection is unable to illustrate that variability,  or the probability of portfolio survivability over a lifetime,  or other outcomes.