Is it possible to have a continuous line with geom_line across facets with facet_nested?

Question

I'm learning how to use geom_line with facet_nested and I cannot produce a continuous line across facets and between each pair of facets there is a gap. What I try to do is to obtain a clean and continuous line as if I produced the very same graphic using Microsoft Excel. Before asking this question here, I searched and I found the following thread in StackOverflow :

How can I draw geom_line across facets or grid

Despite the appearance, this is not exactly what I'm looking for, given that the author doesn't really group the data as facet_nested does. So in order to better see what is my problem, I provide a test case below that you can easily copy/paste in RStudio (Windows environment) in order to understand where is the problem.

Here is the dataframe that I use for my graphic:

df_graph_data = data.frame(
    year = c(
        rep.int("2020", times = 11), 
        rep.int("2021", times = 12), 
        rep.int("2022", times = 3)
    ),
    month_name = c(
        "Feburary", "March", "April", "May", "June", "July",
        "August", "September", "October", "November", "December",
        "January", "Feburary", "March", "April", "May", "June", "July",
        "August", "September", "October", "November", "December",
        "January", "Feburary", "March"
    ),
    month_number = c(
        "02", "03", "04", "05", "06", "07",
        "08", "09", "10", "11", "12", "01",
        "02", "03", "04", "05", "06", "07",
        "08", "09", "10", "11", "12", "01",
        "02", "03"
    ),
    number_of_queries = c(
        484819L, 576697L, 843015L, 925175L,
        1102853L, 889212L, 835706L, 774622L,
        701338L, 850297L, 1046064L, 1273363L,
        958868L, 1088284L, 1151606L, 1666950L,
        2025731L, 2731704L, 2429019L, 3228395L,
        3204915L, 2612807L, 2811946L, 3053788L,
        2589273L, 2305433L
    )
)
###
###
### I add the following variable in order to be able to identify and reference
### each observation in my data.frame uniquely
df_graph_data$rownum = 1:nrow(df_graph_data)

And the graphic that I produced using tha above dataframe

library("tidyverse")
library("ggh4x")
options(device = "windows")
###
###
require(scales)
ggplot(data = df_graph_data) +
    geom_line(mapping = aes(
        x = rownum,
        y = number_of_queries
        ),
        size = 1,
        colour = "blue",
        linetype = "solid"
    ) + 
    scale_x_continuous(
        breaks = seq(
            min(df_graph_data$rownum),
            max(df_graph_data$rownum),
            by = 1L
        ),
        labels = df_graph_data$month_number
    ) +
    scale_y_continuous(
        limits = c(0, max(df_graph_data$number_of_queries) + 1000000L),
        breaks = seq(0, max(df_graph_data$number_of_queries) + 1000000L,
            by = 250000L
        ),
        labels = comma,
        expand = expansion(mult = 0, add = 0)
    ) +
    geom_point(mapping = aes(
        x = rownum,
        y = number_of_queries),
        shape = 15
    ) +
    facet_nested(
        ~ year,
        switch = "x",
        scales = "free_x",
        space = "free_x"
    ) +
    labs(
        x = "Month",
        y = "Number of clients queries",
        caption = "Statistics on queries"
    ) +
    theme(
        strip.placement = "outside",
        axis.text.x = element_text(angle = 0, vjust = 0.5, hjust=0.5),
        axis.title.y = element_text(margin = margin(t = 0, r = 15, b = 0, l = 0)),
        axis.title.x = element_text(margin = margin(t = 15, r = 0, b = 0, l = 0)),
        plot.title = element_text(hjust = 0.5, size = 12),
        plot.caption = element_text(size = 8, hjust = 1),
        plot.caption.position = "plot",
        panel.spacing.x=unit(0.0, "lines"),
        panel.spacing.y=unit(0.0, "lines")
    )

I join here the following screenshot that details more my problem

Is there any solution available in ggplot2 for the two problems indicated in the above screenshot in particular for having a continuous line across facets?

Answer 1

It's possible to add annotations that "connect" multiple facets using grid or patchwork, but I think a simpler approach would be to avoid facets altogether and add the year boxes as a separate plot with the same x range.

I think this could also be done using annotations outside the plot range (e.g. with coord_cartesian(clip = "off")), but this method below worked for me faster.

library(patchwork); library(ggplot2); library(scales)

main_plot <- ggplot(data = df_graph_data) +
  geom_line(mapping = aes(
    x = rownum,
    y = number_of_queries
  ),
  size = 1,
  colour = "blue",
  linetype = "solid"
  ) + 
  
  scale_x_continuous(
    breaks = seq(
      min(df_graph_data$rownum),
      max(df_graph_data$rownum),
      by = 1L
    ),   # let's define x range in coord_cartesian instead 
    labels = df_graph_data$month_number,
    name = NULL
  ) +
  scale_y_continuous(
    limits = c(0, max(df_graph_data$number_of_queries) + 1000000L),
    breaks = seq(0, max(df_graph_data$number_of_queries) + 1000000L,
                 by = 250000L
    ),
    labels = comma,
    expand = expansion(mult = 0, add = 0)
  ) +
  geom_point(mapping = aes(
    x = rownum,
    y = number_of_queries),
    shape = 15
  ) +
  coord_cartesian(xlim = c(0,27), expand = FALSE) + # range should align between plots
  labs(
    x = "Month",
    y = "Number of clients queries"
  ) +
  theme(
    strip.placement = "outside",
    axis.text.x = element_text(angle = 0, vjust = 0.5, hjust=0.5),
    axis.title.y = element_text(margin = margin(t = 0, r = 15, b = 0, l = 0)),
    axis.title.x = element_text(margin = margin(t = 15, r = 0, b = 0, l = 0)),
    plot.title = element_text(hjust = 0.5, size = 12),
    plot.caption = element_text(size = 8, hjust = 1),
    plot.caption.position = "plot"
  ) 


# data for year labels
year_lab <- data.frame(
  from  = c(0,  11.5, 23.5),
  to    = c(11.5, 23.5, 27),
  lab   = 2020:2022,
  y_top = 2E5,  
  y_bot = 0E5
)

year_plot <- ggplot(year_lab) +
  geom_rect(fill = "gray80", color = "gray70",
            aes(xmin = from, xmax = to, ymin = y_bot, ymax = y_top)) +
  geom_text(aes((from+to)/2, y = (y_bot+y_top)/2, label = lab),
            vjust = 0.5, hjust = 0.5) +
  theme_void() +
  coord_cartesian(clip = "off", xlim = c(0,27), expand = FALSE) +
  labs(caption = "Statistics on queries") +
  theme(plot.margin = unit(c(0,0,0,0), "lines"))

main_plot / year_plot +
  plot_layout(ncol = 1, heights = c(10,0.5))