You want to remove tick marks and labels.

To remove just the tick labels, as in Figure 8-14 (left), use theme(axis.text.y = element_blank()) (or do the same for axis.text.x). This will work for both continuous and categorical axes:

p <- ggplot(PlantGrowth, aes(x=group, y=weight)) + geom_boxplot()

p + theme(axis.text.y = element_blank())

Figure 8-14. Left: no tick labels on y-axis; middle: no tick marks and no tick labels on y-axis; right: with breaks=NULL

To remove the tick marks, use theme(axis.ticks=element_blank()). This will remove the tick marks on both axes. (It’s not possible to hide the tick marks on just one axis.) In this example, we’ll hide all tick marks as well as the y tick labels (Figure 8-14, center):