如何用箭头和最大值注释线图？

Question

I am trying to annotate a line plot with an arrow pointing to the highest point in line plot and displaying an arrow and maximum value on the plot. I am using the mtcars dataset as my reference. Below is my code.

e <- df$mpg
ggplot(df, aes(x=e, y=df$hp)) + 
  geom_line() + 
  annotate("segment", color="blue", x=max(e), xend = max(e), y=max(df$hp), 
            yend=max(df$hp), arrow=arrow())

Thanks in advance,

Answer 1

Are you looking for something like this:

labels <- data.frame(mpg = mtcars[which(mtcars$hp == max(mtcars$hp)), "mpg"]+7, hp = mtcars[which(mtcars$hp == max(mtcars$hp)), "hp"],text = paste0("Max value at mpg = ", mtcars[which(mtcars$hp == max(mtcars$hp)), "mpg"], " and hp = ", max(mtcars$hp)))


ggplot(mtcars, aes(mpg, hp))+
    geom_line()+
    geom_text(data = labels, aes(label = text))+
    annotate("segment", 
        x=mtcars[which(mtcars$hp == max(mtcars$hp)), "mpg"]+2,
        xend=mtcars[which(mtcars$hp == max(mtcars$hp)), "mpg"]+.2, 
        y= mtcars[which(mtcars$hp == max(mtcars$hp)), "hp"],
        yend= mtcars[which(mtcars$hp == max(mtcars$hp)), "hp"], 
        arrow=arrow(), color = "blue")

Explanation: In order to annotate with the max, we need to find the position of mpg that is the maximum for hp. To do this we use mtcars[which(mtcars$hp == max(mtcars$hp)), "mpg"] . The which() statement gives us the row possition of that maximum so that we can get the correct value of mpg. Next we annotate with this position adding a little bit of space (ie, the +2 and +.2) so that it looks nicer. Lastly, we can construct a dataframe with the same positions (but different offset) and use geom_text() to add the data label.

Answer 2

An alternative solution using packages 'ggpmisc' and 'ggrepel'. (This code can be easily modified for tagging more than one peak by adjusting the value of span.)

library(ggplot2)
library(ggpmisc)
library(ggrepel)

ggplot(mtcars, aes(mpg, hp))+
  geom_line()+
  stat_peaks(span = NULL,
             geom = "text_repel",
             mapping = aes(label = paste(..y.label.., ..x.label..)),
             x.label.fmt = "at %.0f mpg",
             y.label.fmt = "Max hp = %.0f",
             segment.colour = "blue",
             arrow = grid::arrow(length = unit(0.1, "inches")),
             nudge_x = 5)