Adding a gap in geom_segment in ggplot2 - r

Question

I have some data with id, time_to_event, event, therapy start and therapy stop variables.
Here is a reproducible example:
data <- structure(list(id = structure(c(1L, 3L, 9L, 2L, 5L, 10L, 7L,
8L, 4L, 6L), .Label = c("1", "4", "2", "9", "5", "10", "7", "8",
"3", "6"), class = "factor"), event = c("Death", "Death", "Death",
"Y", "Death", "Y", "Y", "Y", "Death", "X"), time_to_event = c(89L,
83L, 74L, 88L, 78L, 72L, 77L, 76L, 79L, 78L), start = c(18L,
3L, 13L, 16L, 10L, 6L, 20L, 11L, 14L, 9L), stop = c(89L, 83L,
74L, 88L, 78L, 72L, 77L, 76L, 79L, 78L)), row.names = c(NA, -10L
), class = "data.frame")
I computed a plot, which shows the type of event as geom_point and the time of the therapy as geom_segment with start and stop. Here is the code:
colors <- c("X" = "dodgerblue3",
"Y" = "red2",
"Death" = "black") # Defining the colors for the event type
event_symbols <- c("X" = "\u25CF",
"Y" = "\u25CF",
"Death" = "\u2020") # <-- Problem: When I use the shape 84 (T) the code is plotted. When I use "/u2020", I get the above-mentioned error message
five_day_lines <- seq(0, 90, 5)
day_lines <- 1:90
day_lines <- day_lines[!day_lines %in% five_day_lines]
data$id <- factor(data$id, levels = data$id[order(data$time_to_event, decreasing = T)])
ggplot(data = data) +
geom_hline(yintercept = five_day_lines, color = "grey", alpha = .35) +
geom_hline(yintercept = day_lines, color = "grey", alpha = .25) +
geom_segment(aes(x = id, xend = id, y = start, yend = stop), color = "dimgray", size = 1) +
geom_point(aes(x = id, y = time_to_event, shape = event, color = event), size = 3) +
scale_y_continuous(limits = c(0,90), breaks = c(seq(0, 90, 5)), name = "Days after study inclusion") +
scale_x_discrete(name = "ID") +
coord_flip() +
scale_color_manual(values = colors, breaks = c()) +
scale_shape_manual(values = event_symbols, breaks = c("X", "Y", "Death"),
guide = guide_legend(override.aes = list(color = c("dodgerblue3", "red2", "black")))) +
theme(legend.position = "bottom",
legend.title = element_text(size=12, face="bold"),
panel.background = element_blank(),
legend.background = element_blank()) +
labs(color="Medication", shape="Event type")
The plot looks like the following:
My question is, whether it is possible to add a gap in the geom_segment at a certain point, let's say at half of the length of the segments. Ideally, the break should look like a 'typical' gap in a plot axis like the following from https://rstudio-pubs-static.s3.amazonaws.com/235467_5abd31ab564a43c9ae0f18cdd07eebe7.html:
Here, we can see a kind of double slash. I had the idea of taking the double slash unicode character (Link) but this, I think, would simply generate a new layer and would disregard the desired gap or 'interruption' of the geom_segment. Is there a smart way to compute such a gap, where the geom_segment ends at the one slash and starts again at the 2nd slash? Ideally, it works with the double slash but if there is another way with another appropriate shape or something else, I would be fine with that.
I would be grateful, if there is a smart solution for this problem.
Thanks a lot.

Answer 1

I don't think there exists specialised layers in ggplot2 or extensions that deal with gaps specifically. One can of course abuse the arrow argument in geom_segment() to draw a stumpy arrow at the extremes of a segment, but that probably doesn't alleviate the necessity to use a second layer. Below you'll find a simplified example of that.
library(ggplot2)
data <- structure(list(
id = structure(c(1L, 3L, 9L, 2L, 5L, 10L, 7L, 8L, 4L, 6L),
.Label = c("1", "4", "2", "9", "5", "10", "7", "8", "3", "6"),
class = "factor"),
event = c("Death", "Death", "Death", "Y", "Death", "Y", "Y", "Y", "Death", "X"),
time_to_event = c(89L, 83L, 74L, 88L, 78L, 72L, 77L, 76L, 79L, 78L),
start = c(18L, 3L, 13L, 16L, 10L, 6L, 20L, 11L, 14L, 9L),
stop = c(89L, 83L, 74L, 88L, 78L, 72L, 77L, 76L, 79L, 78L)),
row.names = c(NA, -10L),
class = "data.frame"
)
gap_start <- 30
gap_stop <- 50
ggplot(data, aes(x = start, xend = stop, y = id, yend = id)) +
geom_segment(aes(xend = gap_start), arrow = arrow(90, unit(1, "mm"))) +
geom_segment(aes(x = gap_stop), arrow = arrow(90, unit(1, "mm"), ends = "first"))
Created on 2021-07-14 by the reprex package (v1.0.0)