Overlay Polygons in ggplot2 and make the overlay transparent

Question

I'd like to make an overlay of several polygons with ggplot. The overlay's fill should be transparent but their borders should be red. I want to see only the fill of the first polygon so I thought to make the overlays transparent...but I can't get them fully transparent. Somehow I would be easier to just define the fill color as no-fill...but I don't know how to do it. Any ideas?

Here is some code to reproduce an example:

    ids <- factor(c("1.1", "2.1", "1.2", "2.2", "1.3", "2.3"))

    values <- data.frame(
      id = ids,
      value = c(3, 3.1, 3.1, 3.2, 3.15, 3.5)
    )

    positions <- data.frame(
      id = rep(ids, each = 4),
      x = c(2, 1, 1.1, 2.2, 1, 0, 0.3, 1.1, 2.2, 1.1, 1.2, 2.5, 1.1, 0.3,
            0.5, 1.2, 2.5, 1.2, 1.3, 2.7, 1.2, 0.5, 0.6, 1.3),
      y = c(-0.5, 0, 1, 0.5, 0, 0.5, 1.5, 1, 0.5, 1, 2.1, 1.7, 1, 1.5,
            2.2, 2.1, 1.7, 2.1, 3.2, 2.8, 2.1, 2.2, 3.3, 3.2)
    )

    datapoly <- merge(values, positions, by=c("id"))

    p <- ggplot(datapoly, aes(x=x, y=y)) + geom_polygon(aes(fill=value, group=id))
    # overlay the same plot with red borders and transparent fill
    p <- p + geom_polygon(aes(group=id, alpha=1),colour="red",size=1.1)
    p

Answer 1

If you want the second set of geom_polygon to not be filled in, just set the fill to NA.

ggplot(datapoly, aes(x=x, y=y)) + 
  geom_polygon(aes(fill=value, group=id)) +
  geom_polygon(aes(group=id), alpha=1,colour="red", fill=NA, size=1.1)

In this case, you don't need two geom_polygon calls, though

ggplot(datapoly, aes(x=x, y=y)) + 
  geom_polygon(aes(fill=value, group=id), colour="red", size=1.1)