It is. And it is just correlation. It does not, by itself, prove causality. But when you place it next to all sorts of other data points, it begins to paint a picture. Prior to 2009, the Fed's balance sheet had been relatively stable for recent memory.
While I am not disagreeing with you, I am sure you're aware of the common problem of using regressions on trending time series. The results are often spurious and don't necessarily indicate a causal relationship, as you mention yourself. More generally, the example of the BoJ would suggest that the expansion of the CB's balance sheet doesn't always and everywhere translate into higher equity prices. For instance, between 2000 and 2005 BoJ's balance sheet nearly doubled while Nikkei was roughly unchanged. I am very curious about the other data points you're referring to. Again, I am not disagreeing, just interested in the argument.
Yes, there's always a problem with every chart that can be found by the person not willing to have their belief in the Fed shaken. As for other charts, I will post some on Monday. It's Friday evening, and I really need to get out of the office and head home for the little one's footie game. I'll mark this for return on Monday.