Annotate each facet on a qplot separately, when using two variables to facet

Question

I am producing a faceted graph with 4 facets, using two variables each with 2 levels to define facets.

I am using a code like this:

qplot(hp, mpg, data=mtcars, 
  facets=vs~am, size=I(3), geom=c("point","smooth"),
  xlab="Horsepower", ylab="Miles per Gallon")

I am trying to place a unique label onto each facet that says "Slope = the slope of the line"

I have seen a few questions on here that describe how to do this when facets are defiend by one variable, but not 2. How do I go about adding unique labels?

I know I can use geom_text, something like this

geom_text(sig=c("Slope = 1","Slope = 2","Slope = 3","Slope = 4")),aes(x=4.5,y=1,label=sig))

You want to change texts in facet, right? This may not be the best, but how about changing factor names? — jazzurro, Aug 20 '14 at 04:32
I basically want to add a comment onto each facet that is not in the data. — Shannon Hodges, Aug 20 '14 at 04:45

rnso · Accepted Answer · 2014-08-20T05:10:25.170

2

Try:

mtcars$slope = with(mtcars, ifelse(am & vs, 1, ifelse(am,2, ifelse(vs, 3, 4))))

ggplot(data=mtcars)+
    geom_point(aes(x=hp, y=mpg))+
    facet_grid(vs~am)+
    labs(x="Horsepower", y="Miles per gallon")+
    geom_text(aes(x=250,y=50,label=paste("slope=",slope)))

enter image description here

edited Aug 20 '14 at 05:10

answered Aug 20 '14 at 04:02

rnso

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You need to define the values of `vs` and `am` (the facetting variables) in a `data.frame` with the text. – mnel Aug 20 '14 at 04:09
Corrected above. You may take away the downvote now! – rnso Aug 20 '14 at 05:10
Thanks. I am awaiting Shannon's feedback. – rnso Aug 20 '14 at 06:10

Shannon Hodges · Answer 2 · 2014-09-04T04:36:01.457

I also found a method outlied here that allows you to change where one of the labels is placed. Ie not all labes have to be at the same x,y co-ordinates.

mtcars[, c("cyl", "am", "gear")] <- lapply(mtcars[, c("cyl", "am", "gear")], as.factor)

attach(mtcars)

p <- qplot (hp, mpg, facets=vs~am, size=I(3), xlab="hp", ylab="mpg")

len <- length(levels(mtcars$vs)) *  length(levels(mtcars$am))
vars <- data.frame(expand.grid(levels(mtcars$vs), levels(mtcars$am)))
colnames(vars) <- c("vs", "am")
dat <- data.frame(x = 300, y = 50, vars, labs=LETTERS[1:len])

p <- p + geom_text(aes(x, y,
    label=c("Slope = 1","Slope = 2","Slope = 3","Slope = 4"),
    group=NULL), data=dat)

p

To change the location of one specific label:

dat[1, 1:2] <- c(200, 20)   #to change specific locations
p

Annotate each facet on a qplot separately, when using two variables to facet

2 Answers2