tm <- data.frame(Marker = sort(rep(letters[1:5], 10)), x = runif(50), y = runif(50))

p1 + geom_line(data = tm, aes(linetype = Marker, x = x, y = y), alpha = 0) +
  scale_linetype(guide = guide_legend(override.aes = list(alpha = 1)), labels =
                   c(expression(A[1]), expression(A[2]), expression(A[3]), expression(A[4]), expression(A[5]))) +
  theme(legend.position = c(.75, .25))