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.
A description is on
http://www.dormantbitcoin.com/dormantbitcoin/dbchart/.
A bitcoin address is called dormant since its last sending block. Accordingly, all its bitcoins are said to be stayed in that block. If an address has never sent, the first received block is considered as the starting of the dormancy. In the Dormant Bitcoin (DB) chart, the X-axis is the block range, and the Y-axis is the number of bitcoins staying in the corresponding block range.
If an address is dormant for a long time, it is very likely that the owner losts the key, and hence bitcoins. With this chart, the picture of the dormancy of all bitcoins becomes clear. Also, total lost bitcoins could be guessed. Knowing this information is very interesting, since existing bitcoins are more valuable if more bitcoins are lost. As shown in the chart, most of early-age bitcoins are dormant, since miners do not care them due to low values.
In addition, although most of middle-age bitcoins are spent, a small fraction of them is stayed. This information is more important, since it gives the answer on bitcoin lost rate when people cares. It can give us a guidance on how many bitcoins WILL be lost in the future.