What exactly does this chart show? The fraction of addresses that received coins but did not spend them? There is no description on the site. OP if this is your site you need to improve it.

If this is so, it doesn't necessarily mean hoarding. You also need to look at the total number of addresses observed within that period to make a better determination of what happened. For example, people breaking up wallets containing large number of BTC's into multiple smaller wallets would also give you the same effect on the chart.