Periodical comic sales are in freefall, but ICv2 says that in June graphic novel sales were so strong they lifted the entire field to an overall gain:

Sales of graphic novels in June were very strong, less than one-half of one percent below the total of May 2007, which is the highest ever recorded by ICv2 since we have been tracking comic and graphic novel sales.  May 2007’s total was driven by Y: The Last Man Vol. 9, The Walking Dead Vol.6, five Civil War-related titles, DC’s 52 Vol.1, Archaia Studios’ Mouseguard and Dynamite Entertainment’s The Boys.  Not surprisingly June 2008’s sterling performance was powered by a troika of potent releases including Y: The Last Man Vol. 10, Fables Vol. 10 and The Walking Dead Vol. 8.