Expressions from data.frame to ggplot2 legend

Question

I would like to add an expression to a legend entry without entering the legend directly (since I am looping over variables). Essentially I would like this:

d <- data.frame(x=1:10,y=1:10,f=rep(c("0–74",">=75"),each=5))
qplot(x,y,data=d,color=f)   

to output the way this does:

qplot(x,y,data=d,color=f) +
scale_colour_manual(values=1:2,breaks=c("0–74",">=75"),
labels=c(expression(0<=75), expression("">=75)))

(But actually I would like the the first entry 0<=74 to be 0-74, but I'm having trouble mixing expressions and non-expressions.)

I'm sure it's some kind of setting, but everything I've tried hasn't worked. Any ideas?

Answer 1

I think you can do this within your loop by using parse(text=) to convert a string to the appropriate expression. So you could set scale_colour_manual with the appropriate labels by taking the character strings from your f variable and passing them in a manner something like this (some tweaking may be necessary):

scale_colour_manual(...,labels=c(parse(text=lab1),parse(text=lab2)))

Although parse doesn't like ">=75" so you'll probably want something like "''>=75".

For example:

qplot(x,y,data=d,color=f) +
  scale_colour_manual(values=1:2,breaks=c("0–74",">=75"),
  labels=c(parse(text="0-74"), parse(text=paste("''",">=75",sep=""))))