how to make vertical and horizonal line up to the axis boudary in ggplot

Question

I want to use a vertical and horizonal line to mark a point in a facetd plot.

But the vertical and horizonal line can not touch the axis boudary.

I used a simple example to show the problem.

library(tidyverse)
library(ggh4x)
# data prepared
df = data.frame(
  x = c(1, 2, 3, 4, 10, 20, 30 ,40),
  y = c(1, 2, 3, -1, 10 ,20, 30, -10),
  group = c(1, 1, 1, 1, 2, 2, 2, 2)
)

add.point = data.frame(
  group   = c(1, 2),
  x       = c(1, 10),
  y       = c(1.5, 15),
  x_hline = c(0, 0),
  y_hline = c(1.5, 15),
  x_vline = c(1, 10),
  y_vline = c(0, 0)
)

I tried several ways, such as expand_limits() , try scale_x(y)_continuous() and coord_cartesian() , but all failed.

By the way, I can live with the point y < 0 not showing up in the plot.

My question is that is there a way that make vertical and horizonal line touch the axis boudary in ggplot.

(p0 = df %>% 
  ggplot(aes(x = x, y = y)) +
  geom_point() +
  geom_smooth(se = F) +
  facet_wrap(vars(group), scales = 'free')+
  geom_point(data = add.point, aes(x = x, y = y), color = 'red')+
  geom_segment(data = add.point,
               aes(x = x_hline, y = y_hline,
                   xend = x, yend = y), linetype = 'dashed')+
  geom_segment(data = add.point,
               aes(x = x_vline, y = y_vline,
                   xend = x, yend = y), linetype = 'dashed'))


# try expand_limits()
(p1 = p0 + expand_limits(x = 0, y = 0))

# try scale_x(y)_continuous()
(p1 =  p0 + scale_x_continuous(expand = c(0, 0)) + scale_y_continuous(expand = c(0, 0)))


# try coord_cartesian()
(p1 = p0 + coord_cartesian(ylim = 0))

Answer 1

You could use eg -Inf for the endpoints of your segments and use expand_limits to extend the limits to zero:

library(ggplot2)

ggplot(df, aes(x = x, y = y)) +
  geom_point() +
  geom_smooth(se = F) +
  facet_wrap(vars(group), scales = 'free')+
  geom_point(data = add.point, aes(x = x, y = y), color = 'red')+
  geom_segment(data = add.point,
               aes(x = -Inf, y = y_hline,
                   xend = x, yend = y), linetype = 'dashed')+
  geom_segment(data = add.point,
               aes(x = x_vline, y = -Inf,
                   xend = x, yend = y), linetype = 'dashed') +
  expand_limits(x = 0, y = 0)