[R] how to add a legend in a barchart

[ronggui]

i use 
#pic 1
library(lattice)
barchart(V1~V2,data,panel=function(x,y,...){
panel.barchart(x,y,...)
panel.abline(v=6.76,col="red")},
)
to draw a picture.and i want to add a legend to telling others the line vertical is x=6.76.so how should i do? legend does not works.
thank you .
i have use help.search("legend") but no result i want.