The Portfolio Growth chart in the various Portfolio tools plots the full range of a meandering portfolio much like a weather map tracking potential storm paths. Think of it as beginning your portfolio in every historical year at once and watching how they all unfolded. This tool is equally useful for projecting accumulation uncertainty as it is for mapping retirement scenarios.
How To Interpret The Chart
The Portfolio Growth chart is very similar to a traditional line-chart you may find elsewhere that charts the growth of a portfolio over time, but with one major difference. Instead of simply charting returns between a single start and end date, it calculates the performance of every historical start date simultaneously. Every start date is aligned at year zero on the chart, and the resulting spread of returns shows the full range of outcomes regardless of when you were lucky or unlucky enough to invest.
Note that the color of the lines is sorted by start year. Darker lines are from older start years while lighter lines are from more recent start years. Depending on the portfolio, you may also see gray lines. These indicate start years with incomplete data where the returns are only estimated based on the best numbers I have available.
The Portfolio Growth chart also allows you to adjust the settings to model different personal investing scenarios. The starting value sets the leftmost portfolio origin at the desired point, while the annual contribution controls how much additional money from new savings that you invest every year along the way. Negative numbers are displayed in (parentheses) and represent withdrawals from your portfolio that are helpful for modeling retirement. Both contributions and withdrawals are adjusted for inflation and represent the same amount of purchasing power each year.