How do I draw an arrow on a histogram drawn using ggplot2?

Question

Here is dataset:

   set.seed(123)     myd <- data.frame (class = rep(1:4, each = 100), yvar = rnorm(400, 50,30))     require(ggplot2)     m <- ggplot(myd, aes(x = yvar))     p <- m + geom_histogram(colour = "grey40", fill = "grey40", binwidth = 10)  +        facet_wrap(~class) + theme_bw( )      p + opts(panel.margin=unit(0 ,"lines")) 

I want to add labels to bars which each subject class fall into and produce something like the post-powerpoint processed graph. Is there way to do this within R ? ......

Edit: we can think of different pointer such as dot or error bar, if arrow is not impossible

Let's say the following is subjects to be labelled:

class   name       yvar 2       subject4    104.0 3       subject3    8.5 3       subject1    80.0 4       subject2    40.0 4       subject1    115.0  classd <- data.frame (class = c(2,3,3,4,4),   name = c  ("subject4", "subject3", "subject1", "subject2", "subject1"),   yvar = c(104.0, 8.5,80.0,40.0, 115.0)) 

Answer 1

Here is partial solution using geom_text() to add labels and geom_segment() with the arrow option to add arrows.

The drawback is that I had to manually choose y positions for each arrow and label. Maybe someone else can help figure out how programmatically find the histogram bar heights.

set.seed(123) myd <- data.frame (class = rep(1:4, each = 100), yvar = rnorm(400, 50,30))  library(ggplot2) library(grid) # unit() is in the grid package.  arrow_pos = read.table(header=TRUE, stringsAsFactors=FALSE,                        text="class   name       yvar                              2       subject4    104.0                              3       subject3    8.5                              3       subject1    80.0                              4       subject2    40.0                              4       subject1    115.0")  arrow_pos$y = c(3, 5, 9, 13, 1) # Manually enter y position. arrow_pos$class = factor(as.character(arrow_pos$class),     levels=c("1", "2", "3", "4")) # Gets rid of warnings.  p1 = ggplot(myd, aes(x=yvar)) +      theme_bw() +      geom_histogram(colour="grey40", fill="grey40", binwidth=10) +      facet_wrap(~ class) +      opts(panel.margin=unit(0 ,"lines")) +      geom_text(data=arrow_pos, aes(label=name, x=yvar, y=y + 2), size=3) +      geom_segment(data=arrow_pos,                    aes(x=yvar, xend=yvar, y=y + 1.5, yend=y + 0.25),                   arrow=arrow(length=unit(2, "mm")))  png("p1.png", height=600, width=600) print(p1) dev.off()