So that's an average investment of about 136.000 USD/day since inception. While an inflation rate of 3600 BTC/day corresponds to 792.000 USD/day at todays prices. [ ... ] I was just looking at the plots in your graph though, but I shouldn't be off by that much.
No need to read the graphs for that: the number of coins that they held and bought, day by day, was tabulated by @jl2012. See page 1 of this thread. Only the last plot, the Est. Accum. Investment, is not in the table; but it is easily computed from it.