It's the result of what would have happened to that $750K portfolio after one year starting in 1871, 1872, 1873, etc. Some years the investment returns are good and you end up with more than $750K at the end of year 1. Some years the investment returns are poor and you end up with less than $750K at the end of year 1.
Each colored line on the result graph represents how you would have done had you retired in a particular year.
Another way to say it is that the $750K is what you start with at year 0.
"At times the world can seem an unfriendly and sinister place, but believe us when we say there is much more good in it than bad. All you have to do is look hard enough, and what might seem to be a series of unfortunate events, may in fact be the first steps of a journey." Violet Baudelaire.