Unique y-axis in facet_grid in each single panel (rows AND columns)

Question

I am struggling to give each single pannel in facet_grid its own y-axis.

So basically I want all subplots of this plot to have it's own y-axis (so that you can actually see the data). At the same time, I want a fixed x-axis per column as it is right now. The strip labels should also be as it is now.

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I tried to use facet_wrap instead, which will indeed give each single panel its own y-axis. But then I struggle to fix the x-axis for each column. scales="free_y" fixes the x-axis among ALL panels, not per column as in facet_grid. Also, I get strip labels for each subplot, which I also don't want.

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This is the relevant part of the code I use with some sample data that should be sufficient to build a very similar plot:

BAAM = data.frame(Subject = rep(c("S1", "S2", "S3", "S4"), each=4),
                      Feature = rep(c("Maximum negative\namplitude [µV]", "Duration [sec]", "Descending\nslope [µV/sec]", "Ascending\nslope [µV/sec]"), times=4),
                      mean    = c(-200, 1, 1500, 1600, -210, 1.1, 1300, 1500, -250, 3, 1400, 1600, -50, 0.5, 1000, 1000),
                      dif     = c(20, 0.1, 200, 300, 10, -0.3, -100, 400, 30, 0.4, -200, -200, -20, 0.6, 300, 400))

library(ggplot2)
ggplot(BAAM, aes(x=mean, y=dif)) + 
  geom_point() + 
  # facet_grid(rows=vars(Subject), cols=vars(Feature), scales = "free", switch="both") +
  facet_wrap(Subject+Feature~ ., scales="free_y", ncol = 4, strip.position = "bottom") +  
  theme_minimal() +
  ggtitle("Slow Wave Characteristics - Bland Altman plot") +
  annotate("segment", x=-Inf, xend=Inf, y=-Inf, yend=-Inf)+
  annotate("segment", x=-Inf, xend=-Inf, y=-Inf, yend=Inf)+  
  theme(text = element_text(size = 20),
        axis.title.x = element_blank(),
        axis.title.y = element_blank(),
        legend.title = element_blank(),
        axis.text.x = element_text(angle = 45, hjust=1),
        panel.spacing = unit(2, "lines"),
        strip.placement = "outside",
        strip.switch.pad.wrap = unit(0, "lines"))

If this post is a duplicate post I'm very happy to be referred to the respective post. I have done an extensive research though. I found very related problems, but none exactly tried to solve the exact same problem I had. I very much appreciate your help.

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UPDATE 31.08.2019

Thanks to @s_t I discovered the lemon package with the facet_rep_grid function. It's still not exactly what I want, but one step closer. Now the a-xis of all subplots is visible, but the y-limits are still the same per row. I would like the y-limits to be individual for each panel/subplot.

library(lemon)
facet_rep_grid(rows=vars(Subject), cols=vars(Feature), scales = "free", switch="both", repeat.tick.labels = 'y')

Answer 1

A bit late for the party but here is an option which uses facet_wrap and patchwork to achieve your desired result, i.e. I create separate plots for each Subject or row which I then combine using patchwork.

library(ggplot2)
library(patchwork)

BAAM |>
  split(~Subject) |>
  lapply(function(.data) {
    x_title <- unique(.data$Subject)

    remove_strip_x <- if (x_title != "S4") theme(strip.text.x = element_blank())

    ggplot(.data, aes(x = mean, y = dif)) +
      geom_point() +
      labs(y = x_title) +
      facet_wrap(
        facets = vars(Feature),
        scales = "free_y",
        nrow = 1,
        strip.position = "bottom"
      ) +
      annotate("segment", x = -Inf, xend = Inf, y = -Inf, yend = -Inf) +
      annotate("segment", x = -Inf, xend = -Inf, y = -Inf, yend = Inf) +
      theme_minimal() +
      remove_strip_x
  }) |>
  wrap_plots(
    ncol = 1
  ) &
  plot_annotation(title = "Slow Wave Characteristics - Bland Altman plot") &
  theme(
    text = element_text(size = 12),
    axis.title.x = element_blank(),
    legend.title = element_blank(),
    axis.text.x = element_text(angle = 45, hjust = 1),
    panel.spacing = unit(2, "lines"),
    strip.placement = "outside",
    strip.switch.pad.wrap = unit(0, "lines")
  )