Adding a legend to ggplot with facet_grid

Question

I am creating a 3x3 faceted graph using the code shown below. The problem is I get no legend.

# Create column vectors
XID <- rep(c(1,5,10), each=57)
TAD.unit <- c(0, 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 4, 6, 8, 10, 12, 16, 20, 24, 36, 48, 72)
TAD <- rep(TAD.unit, length=length(XID))
FID <-rep(c(1,2,3),each=length(TAD.unit),length=length(XID))
time <- TAD + (FID-1)*14*24
dist1 <- pweibull(TAD,2,2)
dist2 <- pweibull(TAD,2,4)

# Create data frame
data.df <- as.data.frame(cbind(XID,time, FID, dist1, dist2, TAD))

library(ggplot2)
label_both = function(column,value){paste(column,"=",value)}

# Create plot
my.plot1 <- ggplot(data.df, aes(x=TAD, y=dist1)) + geom_point() + 
  geom_line(aes(x=TAD, y=dist2)) +
  facet_grid(XID ~ FID, labeller=label_both) +
  labs(x = "TAD", y = "Response")

# alternative data structure per recommendation in
# http://stackoverflow.com/questions/15418302/ggplot2-how-to-show-the-legend?rq=1
library(reshape)
df.2 <- melt(data.df, id=c("XID","FID","TAD","time"))

I tried using data frame df.2 to see if that helps per the recommendation in the stack overflow thread I provide the link for. I tried various ggplot commands, but I still can't get it to work. Can someone please help me?

Also, how can I position the legend somewhere inside the 3x3 where there is blank space?

Thank you very much!

Answer 1

As for the legend drawing, use your melted dataframe and eg use colour to distinguish your two distance datasets. Eg like this:

ggplot(df.2, aes(x=TAD, y=value, colour = variable)) + geom_line() + facet_grid(XID ~ FID, labeller=label_both) + labs(x = "TAD", y = "Response")

And as for the legend positioning, I would refer you to this question with a good answer:

Position legend in first plot of facet