By Pat Loeb

PHILADELPHIA (CBS) — If you think of Eastern State Penitentiary as just a tourist attraction, you might have to think again.  Its newest exhibit is designed to make a powerful social statement.

The exhibit is called “The Big Graph,” a description of both its size (16 feet tall) and the issue it illustrates.

“You can see the enormous, truly historic increase in the number of Americans that the United States now puts in prison,” says Eastern State Penitentiary vice president Sean Kelly (second from left in photo), who notes that the percentage of the US population in prison has more than quadrupled since the prison closed in 1970.

And, Kelly points out, it has happened without much public debate.

“We’ve always believed Eastern State is the best place to have these conversations, and the Big Graph is by far the most ambitious tool we’ve built yet,” he said.

The graph, rising in the prison yard, also shows the US has by far the highest rate of incarceration and that the growth in the prison population is largely black and Latino.

Former inmates William Harrison and John Toth were on hand to dedicate the graph, which Toth says shows a disturbing trend.

“I think it’s deplorable,” Toth (second from right in photo) said.  “You’ve got people in prison that shouldn’t be in prison.”

A point that prison officials think is at least worth debating.