R - get value of one variable for maximum value of another variable INSIDE a ggplot call-CodePudding

I have some code like this:

seq(0, 2*pi, length.out = 100) %>% 
  cbind.data.frame(t = ., Y = sin(.)) %>% 
  qplot(t, Y, data = ., geom = "line") 
  geom_segment(aes(y = mean(Y), yend = max(Y), x = , xend = ))

I want to create a geom_segment where the arguments x and xend take on the value of t for which Y is highest. I found the question that describes how to find the value of one variable based on the maximum value of the other variable, however, I don't know how to apply that solution so that it can be used within a ggplot call, without saving the object to the environemnt. Any help would be appreciated, thanks.

CodePudding user response：

Use which.max, like in the post the question links to, to find the first maximum of Y and extract the corresponding t value. I have also added the horizontal axis.

suppressPackageStartupMessages({
  library(ggplot2)
  library(dplyr)
})

seq(0, 2*pi, length.out = 100) %>% 
  data.frame(t = ., Y = sin(.)) %>% 
  ggplot(aes(t, Y))  
  geom_line()  
  geom_segment(aes(y = mean(Y), yend = max(Y), 
                   x = t[which.max(Y)], xend = t[which.max(Y)]))  
  geom_hline(yintercept = 0)

^{Created on 2022-05-07 by the reprex package (v2.0.1)}

CodePudding user response：

I am not sure, because the output is strange but try this:

seq(0, 2*pi, length.out = 100) %>% 
  cbind.data.frame(t = ., Y = sin(.)) %>% 
  qplot(t, Y, data = ., geom = "line") 
  geom_segment(aes(y = mean(Y), 
                   yend = which(Y == max(Y)), 
                   x =  which(Y == max(Y)) , 
                   xend = which(Y == max(Y))))

CodePudding user response：

Alternatively to (re)calculating the aesthetics "live" you can do the same with the geom's dataframe. Depending on the extent of data manipulations, this is handy in some situations:

seq(0, 2*pi, length.out = 100) %>% 
  cbind.data.frame(t = ., Y = sin(.)) %>% 
  qplot(t, Y, data = ., geom = "line") 
  geom_segment(data = . %>%
                   mutate(mean_Y = mean(Y)) %>%
                   arrange(Y) %>% ## sort, so that row w highest Y comes last
                   tail(1), ## keep last row (with highest Y)
               aes(y = mean_Y, yend = Y,
                   x = t, xend = t)
)