The Dixie Chicks and their camp will continue their struggle this week to figure out where their own Mason-Dixon Line lies on a map of America's arenas. The reason is that ticket sales for their upcoming road run, the Accidents & Accusations Tour, have not as gone as they expected; basically, in many Southern states and Midwest markets, the Chicks have not been forgiven for perceived political sins of the past. Concert industry watchers say some radio station chiefs are extending their boycott of the group's recordings to include advertising bans for the group's concerts. "Obviously there are political overtones...