Meanwhile I checked for confirmation of a $1000k steal the 15th of June 2011, but found none.
The estimated transaction volume on that and the following days is about 230k for the entire bitcoin system.
See
here.
It is not plausible to me that a thief would risk a reversal of his theft, when all he needs to do is to spend the coins to a new address.
The saying is that about 2000 btc were stolen that day, conjectures about many more stolen coins, e.g. 500k, seem to be widely rejected.
Since the 1000k theft is key to your plot, I wonder if you have a particular source. Anyway, I now like your plot even better, guess why

Thanks for looking into this. The link you posted reports the
estimated transaction volume, where Blockchain.info has
attempted to subtract off the change. We must consider the total output value as the "change" could have gone to the thief too. The total output volume on June 19, 2011 was 1,301,575 BTC (EDIT: and 5,000,000 BTC on the 20th and I don't know the time zones assumed.)
https://blockchain.info/charts/output-volume?showDataPoints=false×pan=all&show_header=true&daysAverageString=1&scale=0&format=csv&address= Furthermore, although unlikely, it may be possible that the robbery didn't happen all at once, but over a few days. When the thieves hit their target, then they set off the alarm bells by faking the huge sale that took out all the bids.
Anyway, I now like your plot even better, guess why

Why? I'm curious.