change position of tick marks of a single graph, using ggplot2

Question

Anybody knows how to draw the X-axis tickmarks at the top of a single plot in R, using ggplot2? I ve been looking at tutorials and mail lists and Rhelp without success.

Answer 1

Use position = 'top'

library(ggplot2)

# Generate some data
df = data.frame(x = 1:10, y = 1:10)

# x-axis breaks
breaks = 1:10

# base plot
p <- ggplot(df, aes(x,y)) + geom_point() +
     scale_x_continuous(breaks = breaks, position = 'top') +
     scale_y_continuous(limits = c(0, 11), expand = c(0,0)) +
     theme_bw()

Original solution available using gtable (with some updating to ggplot 2.1.0). See here

library(ggplot2)
library(gtable)
library(grid)

# Generate some data
df = data.frame(x = 1:10, y = 1:10)

# x-axis breaks
breaks = 1:10

# base plot
p <- ggplot(df, aes(x,y)) + geom_point() +
     scale_x_continuous(breaks = breaks) +
     scale_y_continuous(limits = c(0, 11), expand = c(0,0)) +
     theme_bw() 


# Get ggplot grob
g1 <- ggplotGrob(p)  

## Get the position of the plot panel in g1
pp <- c(subset(g1$layout, name == "panel", se = t:r))

# Title grobs have margins. 
# The margins need to be swapped.
# Function to swap margins - 
# taken from the cowplot package:
# https://github.com/wilkelab/cowplot/blob/master/R/switch_axis.R
vinvert_title_grob <- function(grob) {
  heights <- grob$heights
  grob$heights[1] <- heights[3]
  grob$heights[3] <- heights[1]
  grob$vp[[1]]$layout$heights[1] <- heights[3]
  grob$vp[[1]]$layout$heights[3] <- heights[1]

  grob$children[[1]]$hjust <- 1 - grob$children[[1]]$hjust 
  grob$children[[1]]$vjust <- 1 - grob$children[[1]]$vjust 
  grob$children[[1]]$y <- unit(1, "npc") - grob$children[[1]]$y
  grob
}

# Get  xlab  and swap margins
index <- which(g1$layout$name == "xlab-b")
xlab <- g1$grobs[[index]]
xlab <- vinvert_title_grob(xlab)

# Put xlab at the top of g1
g1 <- gtable_add_rows(g1, g1$heights[g1$layout[index, ]$t], pp$t-1)
g1 <- gtable_add_grob(g1, xlab, pp$t, pp$l, pp$t, pp$r, clip = "off", name="topxlab")

# Get x axis (axis line, tick marks and tick mark labels)
index <- which(g1$layout$name == "axis-b")
xaxis <- g1$grobs[[index]]

# Swap axis ticks and tick mark labels
ticks <- xaxis$children[[2]]
ticks$heights <- rev(ticks$heights)
ticks$grobs <- rev(ticks$grobs)

# Move tick marks
  #  Get tick mark length
  plot_theme <- function(p) {
    plyr::defaults(p$theme, theme_get())
  }
  tml <- plot_theme(p)$axis.ticks.length   # Tick mark length

ticks$grobs[[2]]$y <- ticks$grobs[[2]]$y - unit(1, "npc") + tml

# Swap tick mark labels' margins and justifications
ticks$grobs[[1]] <- vinvert_title_grob(ticks$grobs[[1]])

# Put ticks and tick mark labels back into xaxis
xaxis$children[[2]] <- ticks

# Add axis to top of g1
g1 <- gtable_add_rows(g1, g1$heights[g1$layout[index, ]$t], pp$t)
g1 <- gtable_add_grob(g1, xaxis, pp$t+1, pp$l, pp$t+1, pp$r, clip = "off", name = "axis-t")

# Remove original x axis and xlab
g1 = g1[-c(9,10), ]

# Draw it
grid.newpage()
grid.draw(g1)