Actually yes there is. The wall didn't go down in a day, so between November 9 1989 (when the border opened) and when the wall was demolished (early 90s) artists started to paint on the Eastern side. It's still there, it's called the "East Side Gallery" and some of the most famous Berlin Wall art is actually there, like this very famous one.

If you mean graffiti before 1989... then no. There weren't. Because the wall wasn't just "heavily guarded" as you said. There was a litteral death strip where the guard shoot on sight.