Discontinuous y-axis in inset plot with `ggplot2`

Question

I am trying to insert a Kaplan-Meier inset "zoom" plot into the same KM plot (when Survival curves are very similar to allow more accurate inspection). This is currently used by many journals. Unfortunately, I find creating an inset plot with a discontinuous y-axis non-trivial. I have tried various approaches but unfortunately never got to the desired goal.

First I create the two plots (one with risk table, the inset plot without x/y labels and without risk table).

library(survival)
library(survminer)
library(ggh4x)
library(ggbreak)

# fit model
sfit <- survfit(Surv(time, status) ~ sex, data = colon)

# create main plot with risk table
main_plot <- ggsurvplot(sfit, risk.table=TRUE)

# create inset plot
inset_plot <- ggsurvplot(sfit, risk.table=FALSE, ylim = c(0.4,1), legend = "none", xlab ="", ylab = "")

# transparent background for inset_plot
inset_plot$plot <-  inset_plot$plot +
  theme(
    panel.background = element_rect(fill = "transparent",
                                    colour = NA_character_), # necessary to avoid drawing panel outline
    panel.grid.major = element_blank(), # get rid of major grid
    panel.grid.minor = element_blank(), # get rid of minor grid
    plot.background = element_rect(fill = "transparent",
                                   colour = NA_character_), # necessary to avoid drawing plot outline
    legend.background = element_rect(fill = "transparent"),
    legend.box.background = element_rect(fill = "transparent"),
    legend.key = element_rect(fill = "transparent")
  )

main_plot$plot <- main_plot$plot +
  annotation_custom(grob = ggplotGrob(inset_plot$plot ),   
                    xmin = 1.5,xmax=Inf,ymin = -0.05 ,ymax = 0.7)
main_plot

main_plot

For the inset_plot I want a discontinuous y-axis

There are good answers to the stackoverflow question about discontinuous axes in ggplot2.

Approach 1)

The problem is that the segment label does not appear exactly on the y-axis.

inset_plot$plot +
  annotate("segment", 
           x = c(-200, -100), xend = c(100, 200),
           y = c(0.425, 0.425), yend = c(0.45, 0.45)
           ) +
  coord_cartesian(clip = "off") +
  guides(y = guide_axis_truncated(
    trunc_lower = c(-Inf, 0.45), 
    trunc_upper = c(0.425, Inf)
  ))
#> Coordinate system already present. Adding new coordinate system, which will
#> replace the existing one.

Inset plot error 1

Approach 2)

The easiest solution I found for a discontinous axis with ggplot2 was using the ggbreak library. Unfortunately, this only worked for the single plot. As soon as I inserted it as an inset plot, the discontinuous y axis disappeared.

ggbreak_inset <- inset_plot$plot +
  scale_y_cut(c(0.4), which=c(1, 2), scales=c(10, 1), space = 0.1) +
  scale_y_continuous(breaks = c(0.6, 0.8, 1)) 
#> Scale for y is already present.
#> Adding another scale for y, which will replace the existing scale.
ggbreak_inset 

Although no nice // labeling on y-axis, this works for the single plot. But as soon as this plot is inserted as inset plot, the discontinuous y-axis disappears and the axis scaling becomes erroneous.

new_main_plot <- main_plot  # copy
new_main_plot$plot <- new_main_plot$plot +
  annotation_custom(grob = ggplotGrob(ggbreak_inset ),   
                    xmin = 1.5,xmax=Inf, ymin = -0.05 ,ymax = 0.7)
new_main_plot  # discontinuous y-axis missing, axis scaling incorrect

Inset plot error 2

The ideal figure would look something like below. How can I make this discontinuous axis in an inset plot using ggplot2?

expected plot