That graph shows it as being less toxic than the original ore, which makes sense as it's purified uranium without any of the nasty fission products that are hanging around in the ore.

It gradually approaches the ore level over time, which again makes sense, as the fission products are going to build back up.

The point here is that the uranium is going to decay no matter what you do, whether in the controlled environment of a reactor with some attention paid to proper disposal, or in the completely uncontrolled environment of the original ore bed, where it's quite possibly going straight into the water table.