Adding an additional point to ggplot

This question is a follow-up to this post: previous post

I have 28 variables, M1, M2, ..., M28, for which I compute certain statistics x and y.

 library(ggplot2)
 df = data.frame(model = factor(paste("M", 1:28, sep = ""), levels=paste("M", 1:28, sep = "")), a = runif(28, 1, 1.05), b = runif(28, 1, 1.05))

 levels = seq(0.8, 1.2, 0.05)

Here is the plot:

 ggplot(data=df) + 
   geom_polygon(aes(x=model, y=a, group=1), color = "black", fill = NA) + 
   geom_polygon(aes(x=model, y=b, group=1), color = "blue", fill = NA) +
   coord_polar() + 
   scale_y_continuous(limits=range(levels), breaks=levels, labels=levels) +
   theme(axis.text.y = element_blank(), axis.ticks = element_blank(), axis.title.x = element_blank(), axis.title.y = element_blank())

I would like to add a point the the plot, with y-value = 1 for M1 (model1). I tried adding:

 geom_point(aes(y = 1, x = "M1"), color = "red", cex = 0.5)

but it doesn't work. Any idea what I am doing wrong?

Thanks for your help!