title: How can I suppress the vertical gridlines in a ggplot2 plot?
body: <p>I am building a bar chart for which bars suffice as indications of horizontal (x) placement, so I'd like to avoid drawing the superfluous vertical gridlines.</p>
  
  <p>I understand how to style the minor and major gridlines in opts(), but I can't for the life of me figure out how to suppress just the vertical gridlines.</p>
  
  <pre><code>library(ggplot2)
  
  data &lt;- data.frame(x = 1:10, y = c(3,5,2,5,6,2,7,6,5,4))
  
  ggplot(data, aes(x, y)) +
    geom_bar(stat = 'identity') +
    opts(
      panel.grid.major = theme_line(size = 0.5, colour = '#1391FF'),
      panel.grid.minor = theme_line(colour = NA),
      panel.background = theme_rect(colour = NA),
      axis.ticks = theme_segment(colour = NA)
    )
  </code></pre>
  
  <p>At this point, it's looking like I'm going to have to suppress all of the gridlines and then draw them back in with geom_hline(), which seems like kind of a pain (also, it's not entirely clear how I can find the tick/major gridline positions to feed to geom_hline().)</p>
  
  <p>Any thoughts would be appreciated!</p>