It is somewhat difficult to track past evaluations and see where you were, so my suggestion is kind of simple: add a new graph that shows everything relative to the investment.
A single straight line in the middle represents the base (investment) and then have lines above it for + and below it for -. Can be based on either percentage or raw sum, doesn’t matter. Additional investments (made monthly, for instance) would be blended seamlessly into the graph (with small hitches in + - bars).
example:
