Using assign within ggplot loop gives incorrect plots

Question

I'm creating three plots in a loop over I and using assign to save each plot. The y variable is scaled by the loop index. The scaling should ensure that the final panel of plots each has y going from 0 to 1. This isn't happening and the plots seem to be being changed as the loop runs. I'd be grateful if someone could explain this apparently odd behaviour.

library(dplyr)
library(ggplot2)
library(gridExtra)

loci = c(1,2,3)
x <- seq(0,1,0.01)

df <- expand.grid(x = x, loci = loci)
df <- df %>% mutate(y = loci * x)

cols = c("red", "blue", "green")

for (i in loci){
  
  plot_this <- df %>% filter(loci == i)
  
  my_plot = ggplot(plot_this) +
    geom_point( aes( x = x, y = y/i), colour = cols[i]) +
    ylim(0,3) + ggtitle(paste0("i = ", i))
  
  assign(paste0("plot_", i), my_plot)
  print(plot_1)
}

grid.arrange(plot_1, plot_2, plot_3, ncol = 3)

Answer 1

It's due to the lazy evaluation nature of ggplot, and more explanation can be found in this post.

"Looping" with lapply avoids the problem.

Data

library(ggplot2)
library(gridExtra)
library(dplyr)

loci = c(1,2,3)
x <- seq(0,1,0.01)

df <- expand.grid(x = x, loci = loci)
df <- df %>% mutate(y = loci * x)

cols = c("red", "blue", "green")

Code

my_plot <- lapply(loci, function(i) {
  df %>% 
    filter(loci == i) %>% 
    ggplot() + 
    geom_point(aes(x = x, y = y/i), colour = cols[i]) + 
    ylim(0,3) + 
    ggtitle(paste0("i = ", i))
  })

grid.arrange(my_plot[[1]], my_plot[[2]], my_plot[[3]], ncol = 3)

^{Created on 2022-04-26 by the reprex package (v2.0.1)}