I'm looking around online to see if I can find something for you, but the chart you're speaking of I know is in the Unofficial Guide, as well as Passporter.
But generally, the least crowded times of year are January, after New Year's, through President's Day. After President's Day, the crowds dissipate until Easter, or Spring Break. March/April is usually an iffy time because crowd levels depend on when Easter falls, and when schools around the country have spring break.
May is usually pretty quiet.
It starts to pick up again in June, and the heaviest summer crowds hit in July. They then taper off somewhat through August.
Right after labor day, attendance takes a nose dive again. Crowds pick up somewhat through October and November, right up until Thanksgiving week. After Thanskgiving, the crowds nose-dive again until Christmas. THe crowds stay high until after new Year and the whole cycle starts again.
