ggplot2 labeling graphs in a loop

Question

I would like to put vertical lines and labels for the vertical lines for each graph in a loop. the position of the line is saved in a dataframe "grid".

Although the position of the line seems correct the label value and its position is off. My question is why.

    library(ggplot2)
    library(grid)
    library(gridExtra)
    
    
    plots <- list()
    
    grid <- data.frame(x=seq(4), y=c(200, 400, 600, 800))
    
    for (i in 1:4) {
      V1 <- rnorm(1000)
      V2 <- seq(1000)
      df <- data.frame(V1, V2)
      
      plots[[i]] <- ggplot(df, aes(x= V2, y=V1)) +
        geom_point() +
      geom_vline(xintercept = grid[i,2], color="red")+ 
      geom_text(aes(x=grid[i,2], label=grid[i,2], y=3))
    
    }
    
    
    grid.arrange(grobs=plots, nrow=2)

hi, for me the trick was to set. inherit.aes=FALSE in geom_text. I tried to figure out the WHY too, but I could not figure out a good answer to it. If you print the plot in each iteration, the plots are perfect. If you use your code in a lapply (instead of plots[[i]] just plots) it works too. So it seems like geom_text overrides all previous list elements in the for loop ... which seems odd. And even stranger that the rest of the plot is not affected ... if anybody has a good answer to the WHY, please share :) — sambold
– sambold, Commented Jun 27, 2020 at 13:02
You really need to consider using facets. Loops are not the best practice in R, most of the times specially when a vectorized solution is available. — M--
– M--, Commented Jun 30, 2020 at 16:14

Peter H. · Accepted Answer · 2020-06-26 11:59:01Z

2

Ideally you should use annotate instead. The following code works as expected.

library(ggplot2)
library(grid)
library(gridExtra)


plots <- list()

grid <- data.frame(x=seq(4), y=c(200, 400, 600, 800))

for (i in 1:4) {
  V1 <- rnorm(1000)
  V2 <- seq(1000)
  df <- data.frame(V1, V2)
  
  plots[[i]] <- ggplot(df, aes(x= V2, y=V1)) +
    geom_point() +
    geom_vline(xintercept = grid[i,2], color="red")+ 
    annotate("text", x=grid[i,2], label=grid[i,2], y=3)
  
}


grid.arrange(grobs=plots, nrow=2)

^{Created on 2020-06-26 by the reprex package (v0.3.0)}

answered Jun 26, 2020 at 11:59

Peter H.

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Comments

tjebo · Accepted Answer · 2020-06-26 12:18:04Z

I'd take a different approach, using facets.

I'd also probably calculate the sample outside the facet/loop (second option)

library(ggplot2)
grid_df<- data.frame(x=1:4, y=c(200, 400, 600, 800))

ggplot(grid_df) +
  geom_vline(aes(xintercept = y), color="red") +
  geom_text(aes(label = y, x = y, y = Inf), vjust = 1) +
  stat_function(
    inherit.aes = FALSE,
    fun = rnorm, 
    n = 600,
    alpha = 0.2,
    geom = "point"
  )+
  facet_wrap(~x, nrow = 2)


## or, if you want always the same sample in each facet: 
set.seed(42)
df_rnorm <- data.frame(V1 = rep(rnorm(1000), 4), V2 = rep(seq(1000), 4), x = rep(1:4, each = 1000))

ggplot(grid_df) +
  geom_vline(aes(xintercept = y), color="red") +
  geom_text(aes(label = y, x = y, y = Inf), vjust = 1) +
  geom_point(data = df_rnorm, aes(V2, V1), alpha = 0.2)+
  facet_wrap(~ x, nrow = 2)

^{Created on 2020-06-26 by the reprex package (v0.3.0)}

sambold · Accepted Answer · 2020-06-28 10:53:08Z

I had to experiment a little bit to adress the why part of your question. And probably someone else could enrich my explaination with a little bit more expert knowledge and facts (I am just answering based on my gut feeling :D)

As there seems to be no problem, if you print the plots inside the loop or if you print them outside the loop (within another loop that iterates 1:4), I think plot[[i]] only stores the syntax to produce the plots. If you execute plot[[i]] outside of the loop, it uses grid[i,2] to assign label and position of the geom_text part. Outside of the loop i is 4 and thats why all the plots use the same position/label (800).

But I don't really understand why geom_vline does not behave the same way or why aes.inherit or the use of lapply fixes the problem ... If anyone else could shed some light on this, I would appreciate it :)

        library(ggplot2)
        library(grid)
        library(gridExtra)
        
        plots <- list()
        
        grid <- data.frame(x=seq(4), y=c(200, 400, 600, 800))
        
        for (i in 1:4) {
            V1 <- rnorm(1000)
            V2 <- seq(1000)
            df <- data.frame(V1, V2)
            
            plots[[i]] <- ggplot(df, aes(x= V2, y=V1)) +
                geom_point() +
                geom_text(aes(x=grid[i,2], label=grid[i,2], y=3)) +
                geom_vline(xintercept = grid[i,2], color="red") 
                
            
            print(plots[[i]]) # not part of the original syntax
            
        }
        
        
        # leads to wrong position of geom_text
        plots
    
        # leads to wrong position of geom_text
        grid.arrange(grobs=plots, nrow=2)
        
        # correct position of geom_text
        for (i in 1:4){
            print(plots[[i]])
        }
        
        # new grid values also affect plots
        grid <- data.frame(x=seq(4), y=c(2000, 4000, 6000, 8000))
        
        # leads to wrong position of geom_text
        plots

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