ggplot varying length tickmarks

variable_tickmark_length.R

library(tidyverse)

#------------------------------------
# Have x-axis labels offset slightly for clarity using guide_axis(n.dodge = 2),
# but also have different length tickmarks to make  it look nicer. 
# See https://twitter.com/ecologyofgavin/status/1344102509585997824
# Derived from https://stackoverflow.com/a/51312611/6615512
# Requires ggplot 3.0.0 or greater
#------------------------------------


# Some data to plot
lizards = tribble(
  ~group, ~n,
  'Ag',   5,
  '0-3',  8,
  '8-10', 7.5,
  '12-15',8,
  '20-32',7.8,
  'Natural',8.1
)
# put groups in order defined above
lizards$group = forcats::fct_inorder(lizards$group)

# Tick mark lengths adjusted here
top_y_pos = 2
bot_y_pos = 0.8
custom_ticks = tribble(
  ~group, ~y_end,
  'Ag',     top_y_pos,
  '0-3',    bot_y_pos,
  '8-10',   top_y_pos,
  '12-15',  bot_y_pos,
  '20-32',  top_y_pos,
  'Natural',bot_y_pos
)

ggplot(lizards, aes(x=group,y=n)) + 
  geom_point(aes(color=group),size=8) + 
  scale_color_viridis_d() +
  scale_x_discrete(guide = guide_axis(n.dodge = 2)) + 
  geom_linerange(data = custom_ticks, aes(x=group, ymax=2.7, ymin=y_end), # The custom tickmarks
                 size=2,
                 inherit.aes = F) + 
  theme_bw(50) + 
  coord_cartesian(clip='off', ylim=c(3,10)) + # clip off allow geoms, here the tickmarks, to be drawn outside the plots
  theme(legend.position = 'none',
        axis.title = element_blank(),    
        panel.border = element_rect(color='black'), # make the border and y ticks match the custom ticks
        axis.ticks.y = element_line(color='black'),
        axis.ticks.x = element_blank()) # turn offf the regular tickmarks

Creates the following