The lines are shifted a bit so they seem to overlap more than they normally would. The reasoning is likely that someone who was exposed to lead as a child would be more likely to commit violent crimes as an adult.
The question now is whether the relationship is spurious, and there is a different factor that drives the change - perhaps something that drives both independently. But researchers have isolated a molecule in lead that reduces IQ. Does that also increase the likelihood that someone will become a violent criminal? If so, that's your link.
Over the past few decades, laws have been passed limiting the amount of lead in the environment. Could these be taking effect? The prison population has been reduced recently, and there has a been a long term trend downward in the percent of people going to prison. A commentator calls this one of the major unreported public policy stories of the year.
Many factors are associated with this shift, but there's a definite shift in the tendency of younger males - the one's likely to commit violent crimes - to be sentenced to prison. Something seems to have shifted about 35 years ago - which was about the time leaded gasoline and lead paint were phased out.
