Thank you everyone. This is very helpful to refine my thinking. For some reason, this question has been bugging me for a while.

I agree with the technical definition that the average rate of growth does not change over time. The closest way to reframe the question was something

@roomtempmayo wrote:

when a portfolio starts generating "real money" (scare quotes around deliberately subjective term) on its own.

I think I am getting to getting some closure on this now.

There were also couple of more technical thoughts but I realize that my approach was wrong. It's really good to talk things over rather than stew on them in my head.

1) Given that I reinvest the dividends, the growth of the portfolio should not be linear. Right now it roughly does look linear because it consists mostly of my contributions. I am aware that distribution of dividends should cause stock price to drop at the time so it should not matter but I am not fully sure what the impact of reinvestment would be on portfolio growth.

2) The compounding formula for the single $1 is (1+0.07)^t assuming 7% return. Mathematically, this graph looks like the one I posted. Yes, time will be the exponent, I am well aware of it. But I extrapolated that the whole portfolio chart would follow the same pattern.

However, if I am thinking about multiple contributions, I get the sum of various X1*(1+r1)^(time for this compunding) + X1*(1+r2)^(time for this compunding) + .... etc. So perhaps it is unreasonable to expect the smooth plot out of my portfolio.

Anyway.

Maybe the better way to think about inflection is what has been suggested in this thread: think about market gains and what I could have bought with them rather than staring at the chart :D My portfolio gains would have been enough to pay for a trip to Japan which I really would like to take...

@vand But in a real life case study many high-earning + high-saving types here are probably getting to FI (ie 4% WR) mainly from their own efforts (ie money they put in themselves) rather than from the compounding of the money they put it.

Dammit. I thought I could just put in $100k and be done. :D