Define alpha using ggplot to highlight single points

Question

Reproducible code:

library(ggplot2)
library(data.table)

x = rnorm(10000, 0, 1)
y = rnorm(10000, 0, 1)

dat = data.table(x,y)
dat[,id := as.character(1:.N)]

ALPHA = 0.2 

dat[,alpha := ifelse(id == 42, 1, ALPHA)]
alpha2 = unique(dat[,list(id, alpha)])
alpha = alpha2[,alpha]
names(alpha) = dat[,id] 

u = ggplot(dat, aes(x, y, alpha = id)) + geom_point()
u = u + scale_alpha_manual(values = alpha, guide = FALSE) 
u

Output:

Issue:

The point with id = 42 is not highlighted, and continuous alpha-values are still applied to all observations. I would like to see only the point with id = 42 having an alpha of 1 and all the rest with a fixed value of alpha = 0.2.

Answer 1

Another option is to use annotate. Since the visibility is not very good when using alpha = 0.2, , I colored it red (but would work with an alpha argument too).

library(ggplot2)
library(data.table)

x = rnorm(10000, 0, 1)
y = rnorm(10000, 0, 1)

dat = data.table(x,y)
dat[,id := as.character(1:.N)]

alphapoint <- 42

ggplot(dat, aes(x, y)) + 
  geom_point(alpha = 0.2) +
  annotate("point",
           dat$x[alphapoint],
           dat$y[alphapoint],
           color = "red",
           size = 4)

Using alpha:

ggplot(dat, aes(x, y)) + 
  geom_point(alpha = 0.05) +
  annotate("point",
           dat$x[alphapoint],
           dat$y[alphapoint],
           alpha = 1)