How to prevent gridExtra::grid.arrage from automatically scaling the x axis in R?

Question

I run each of the below plots within the loop with no problem, but for some reason, when I try to combine them in grid.arrange the axes get scaled automatically. I can't figure out how to not automatically scale. I literally just want to combine the ggplots together (as if I used par(mfrow = c(2,3)) with base R).

I think the fact that the X axis is really a re-formatted datetime (as.posixct) is throwing it off. Ideally, I would like the X axis to display just the hour.

library(ggplot2)
library(bp)

df <- data(hypnos_data)

ids <- unique(df$ID)
id_tab <- cbind(ids, c(1:5))

index <- c(8:23, 0:7)

plot_list <- list()
for(i in ids){
  
  subs <- df[which(df$ID == i & df$VISIT == 1),]
  subs$DATE.TIME <- as.POSIXct(subs$DATE.TIME)
  subs$hour_rec <- lubridate::hour(subs$DATE.TIME)
  
  subs$hour_rec <- factor(subs$hour_rec, levels = as.character(index), ordered = T)
  
  row.names(subs) <- NULL
  
  tmp <- subs %>% dplyr::filter(WAKE == 0)
  min(tmp$hour_rec)
  
  p <- ggplot2::ggplot(subs, ggplot2::aes(x = DATE.TIME, y = SYST)) +
    
    geom_rect(aes(xmin = min(tmp$DATE.TIME), xmax = max(tmp$DATE.TIME), ymin = -Inf, ymax = Inf),
              fill = "navajowhite2", alpha = 0.03) + 
    
    ggplot2::geom_point(aes(y = SYST), col = 'blue') + 
    ggplot2::geom_smooth(aes(y = SYST), method = "loess", col = 'blue') + 
    ggplot2::geom_point(aes(y = DIAST), col = 'red') + 
    ggplot2::geom_smooth(aes(y = DIAST), method = "loess", col = 'red') + 

    scale_x_datetime(date_label = "%H:%M", date_breaks = "1 hour") +
    theme(axis.text.x = element_text(angle=45)) +
    
    ggtitle( paste("BP Profile for Subject: ", i, sep = "") )
  
  plot_list[[match(i, id_tab)]] <- p
  print(p)
  
  #grid::grid.newpage()
}

The above code should produce 5 plots similar to this (which are correct):

individual plot output

gridExtra::grid.arrange(grobs = plot_list, ncol = 2 )

However, trying to combine them using grid.arrange yields the following:

grid.arrange plot output

Answer 1

The issue is not in the grid.arrange It is due to the reference variable in ggplot2. Some of the axis data is reference values instead of stored values inside the plot. This result in the change in axis when the subs & tmp change for each for-loop interation. Here is a way to walk around it using eval(substitute(...))

library(ggplot2)
library(bp)
library(doParallel)
#> Loading required package: foreach
#> Loading required package: iterators
#> Loading required package: parallel

data(hypnos_data)
df <- hypnos_data
ids <- unique(df$ID)
id_tab <- cbind(ids, c(1:5))

index <- c(8:23, 0:7)

plot_list <- list()
plot_list <- foreach(i = ids) %do% {
  eval(substitute({
    subs <- df[which(df$ID == i & df$VISIT == 1),]
    subs$DATE.TIME <- as.POSIXct(subs$DATE.TIME)
    subs$hour_rec <- lubridate::hour(subs$DATE.TIME)
    
    subs$hour_rec <- factor(subs$hour_rec, levels = as.character(index), ordered = T)
    
    row.names(subs) <- NULL
    
    tmp <- subs %>% dplyr::filter(WAKE == 0)
    min(tmp$hour_rec)
    
    p <- ggplot2::ggplot(subs, ggplot2::aes(x = DATE.TIME, y = SYST)) +
      
      geom_rect(aes(xmin = min(tmp$DATE.TIME), xmax = max(tmp$DATE.TIME), ymin = -Inf, ymax = Inf),
        fill = "navajowhite2", alpha = 0.03) + 
      ggplot2::geom_point(aes(y = SYST), col = 'blue') + 
      ggplot2::geom_smooth(aes(y = SYST), method = "loess", col = 'blue') + 
      ggplot2::geom_point(aes(y = DIAST), col = 'red') + 
      ggplot2::geom_smooth(aes(y = DIAST), method = "loess", col = 'red') + 
      
      scale_x_datetime(date_label = "%H:%M", date_breaks = "1 hour") +
      theme(axis.text.x = element_text(angle=45)) +
      
      ggtitle( paste("BP Profile for Subject: ", i, sep = "") )
  }, list(subs = as.name(paste0("subs_", i)), tmp = as.name(paste0("tmp_", i)))))
  
  p
}

gridExtra::grid.arrange(grobs = plot_list)
#> `geom_smooth()` using formula 'y ~ x'
#> `geom_smooth()` using formula 'y ~ x'
#> `geom_smooth()` using formula 'y ~ x'
#> `geom_smooth()` using formula 'y ~ x'
#> `geom_smooth()` using formula 'y ~ x'
#> `geom_smooth()` using formula 'y ~ x'
#> `geom_smooth()` using formula 'y ~ x'
#> `geom_smooth()` using formula 'y ~ x'
#> `geom_smooth()` using formula 'y ~ x'
#> `geom_smooth()` using formula 'y ~ x'

^{Created on 2021-03-31 by the reprex package (v1.0.0)}