It would be interesting to graph the valuations of all of these companies over time. You could make the x-axis "company age" rather than time in order to make a more apples-to-apples comparison between older YC companies and younger YC companies. I did a bit of work along these lines a few weeks ago:
https://github.com/JesseAldridge/yc_still_good
Here is the graph I'm talking about: https://github.com/JesseAldridge/yc_still_good/blob/master/r...