It's faulty logic, because you can't isolate the impact of YC has the sole contributory variable for the amount of Series A. Otherwise, you're asserting that correlation implies causation.

There are many other contributory factors in the raising of a Series A and why the amount has increased over the years, one major one of which is the fact that the US economy has vastly improved since the Great Recession of 2008 and investor confidence has rebounded. (notice the huge dip in the year of 2008)

Chart annotations usually indicate events which have a causal, identifiable effect on the data that the user would otherwise question as odd without an explination, which is why adding the annotations only confuses your point.