r - How do I make my axis ticks face Inwards in ggplot2

Question

I have made a barplot using ggplot2 and the Journal I need to submit to requires that the axis ticks face inwards.

This is the text representation of my data (dput)

Mean.Inc.melt<-structure(list(Var1 = structure(c(1L, 2L, 1L, 2L, 1L, 2L), .Label = c("Harvest","Pre-Harvest"), class = "factor"), Var2 = structure(c(1L, 1L, 2L, 2L, 3L, 3L), .Label =c("Dip A", "Trip A", "Trip B"), class = "factor"), value = c(2, 34, 1, 36, 3, 46)), .Names =c("Var1", "Var2", "value"), row.names = c(NA, -6L), class = "data.frame")

Including the standard error

SEM.Inc.melt<-structure(list(Var1 = structure(c(1L, 2L, 1L, 2L, 1L, 2L), .Label = c("Harvest", "Pre-Harvest"), class = "factor"), Var2 = structure(c(1L, 1L, 2L, 2L, 3L, 3L), .Label = c("Dip A", "Trip A", "Trip B"), class = "factor"), value = c(1, 12, 1, 2, 1, 6)), .Names = c("Var1", "Var2", "value"), row.names = c(NA, -6L), class = "data.frame")

This is the script I have used so far to create the plot:

ggplot(Mean.Inc.melt,aes(x=Var2,y=value,fill=Var1))+
  geom_bar(stat='identity',position=position_dodge(),colour='black')+
  scale_fill_manual(values=c('#000000','#FFFFFF'))+
  geom_errorbar(aes(ymin=Mean.Inc.melt$value-SEM.Inc.melt$value,
                    ymax=Mean.Inc.melt$value+SEM.Inc.melt$value),width=.1,
                    position=position_dodge(.9))+
  xlab('Treatment')+
  ylab('Percentage Incidence (%)')+
  ylim(0,60)+
  scale_y_continuous(expand=c(0,0),limits=c(0,60))+
  scale_x_discrete(expand=c(0,0))+
  theme_bw()+
  theme(axis.line=element_line(colour='black'),panel.grid.major=element_blank(),
        panel.grid.minor=element_blank(),panel.border=element_blank(),
        panel.background=element_blank())+
  geom_vline(xintercept=0)+theme(legend.position='none')

I guess the point is - does anyone know if there a way I can get my axis to face inwards?

Answer 1

While I don't understand journals' desire to have tick marks on the inside, it's quite straightforward to achieve this with ggplot.

The axis.ticks.length argument to theme allows you to set the length of tick marks. If this is set to a negative value, tick marks will be plotted inwards. For example (reposting a solution by Dennis Murphy here):

library(ggplot2)
library(grid)
ggplot(mtcars, aes(disp, mpg)) + geom_point() + 
  theme(axis.ticks.length=unit(-0.25, "cm"), axis.ticks.margin=unit(0.5, "cm"))

As in the example above, you need to adjust the positioning of the tick labels as well, with axis.ticks.margin.

Note that the value should be passed as a unit object, which requires that the grid package (pre-installed with R) is loaded.