How can I suppress the vertical gridlines in a ggplot2 plot?

As of ggplot2 0.9.2, this has become much easier to do using “themes.” You can now assign themes separately to panel.grid.major.x and panel.grid.major.y, as demonstrated below.

#   simulate data for the bar graph
data <- data.frame( X = c("A","B","C"), Y = c(1:3) )    

#   make the bar graph
ggplot( data  ) +
    geom_bar( aes( X, Y ) ) +
    theme( # remove the vertical grid lines
           panel.grid.major.x = element_blank() ,
           # explicitly set the horizontal lines (or they will disappear too)
           panel.grid.major.y = element_line( size=.1, color="black" ) 
    )

The result of this example is quite ugly looking, but it demonstrates how to remove the vertical lines while preserving the horizontal lines and x-axis tick-marks.