What does this chart tell you?

It might tell you that putting more people in jail has lowered the crime rate. However, as you can see the crime rate has been going down but the incarceration rate has continued to rise. In 36 states, the prison population has more than tripled as a share of state population since 1978. 

The chart and a lot of other significant analysis told the Brennan Center for Justice that while rising incarceration rates helped reduce property and violent crime rates in the 1990s, the effect was much smaller than some other studies have suggested, accounting for 0-10 percent of the total decline over the decade. Since 2000, rising incarceration rates account for less than 1 percent of the decline in crime rates.