axis_break.R

library(ggplot2)
library(grid)


element_box_break <- function (fill=NULL, colour = NULL, linewidth = NULL, linetype = NULL, lineend = NULL, 
                                color = NULL, arrow = NULL, inherit.blank = FALSE) {
  if (!is.null(color)) 
    colour <- color
  if (is.null(arrow)) 
    arrow <- FALSE
  structure(list(fill=fill, colour = colour, linewidth = linewidth, linetype = linetype, 
                 lineend = lineend, arrow = arrow, inherit.blank = inherit.blank), 
            class = c("element_box_break","element_rect", "element"))
}

element_grob.element_box_break <- function(element, label="", ...)  {
  
  start <- unit(0,'npc')
  end <- unit(1,'npc')
  space <- unit(4,'mm')
  gap <- unit(3,'mm')
  s <- segmentsGrob(x0 = unit.c(start,end,end,start,start), 
                    x1 = unit.c(end,end,start,start,start),
                    y0 = unit.c(start,start,end,end,start+space), 
                    y1=unit.c(start,end,end,start+space+gap,start), 
                    gp=gpar(col='black')) # c("red","blue",'green','gold')
  grobTree(rectGrob(gp=gpar(fill=element$fill, col=NA)), s)
}

element_line_break <- function (colour = NULL, linewidth = NULL, linetype = NULL, lineend = NULL, 
          color = NULL, arrow = NULL, inherit.blank = FALSE) {
  if (!is.null(color)) 
    colour <- color
  if (is.null(arrow)) 
    arrow <- FALSE
  structure(list(colour = colour, linewidth = linewidth, linetype = linetype, 
                 lineend = lineend, arrow = arrow, inherit.blank = inherit.blank), 
            class = c("element_line_break","element_line", "element"))
}

element_grob.element_line_break <- function(element, label="", ...)  {
  
  start <- unit(0,'npc')
  end <- unit(1,'npc')
  space <- unit(4,'mm')
  gap <- unit(3,'mm')
  s <- segmentsGrob(x0 = unit.c(end - gap,end - gap, end, end), 
                     x1 = unit.c(end + gap, end + gap, end, end),
                     y0 = unit.c(start+space-0.5*gap,start+space+0.5*gap,start,start+space+gap), 
                     y1=unit.c(start+space+0.5*gap,start+space+1.5*gap,space,end), 
                    gp=gpar(col='black')) # c("red","blue",'green','gold')
  s
}

set.seed(1234)
d <- data.frame(x=1:10,y=runif(10, 12,13))

theme_set(theme_bw())
ggplot(d,aes(x,y)) +
  geom_point() +
  # coord_cartesian(ylim=c(11.94,max(d$y))) +
  theme(axis.line.y = element_line_break(),
        panel.background = element_box_break(fill='grey98'),
        panel.border = element_box_break(fill=NA))

Would be tricky to do in ggplot2: first, need to replace element_line in axis.line to draw the broken axis, replace panel.background and panel.border to skip the gap—then the hard part: add absolute space below/left after training scales, axis breaks that avoid clashes, support for secondary axes.