Several qqtrots on a gragh and a single abline ggplot2 R

Borrowing the QQ plot example: More than two data, how can we implement it in Ggplot2? I need it an abline only.

Sample of data:

set.seed (10)
dat <- data.frame (observed = rnorm (20), sim1 = rnorm (20), sim2 = rnorm (20), sim3 = rnorm (20), sim4 = rnorm (20), sim5 = rnorm (20), sim6 = rnorm (20)

sim1 ... sim6 should have different colors.

All suggestions are welcome.