title: One directional error bars in ggplot2
body: <p>I am plotting some data from a test that was repeated pre and post an intervention. As such I would like to plot the results on the same figure with error bars. As the error bars of the two trials cross, making it difficult to decipher, I would like to plot the error bars upwards only for the post test and downwards only for the pre test.
  Is this possible in ggplot2?</p>
  
  <p>Example code:</p>
  
  <pre><code>library("ggplot2")
  set.seed(1)
  dat &lt;- data.frame(Trial = c(rep("Pre",9),rep("Post",9)), 
                    Time = rep.int(seq(0,120,15),2), 
                    Insulin = c(rnorm(9,15,2),rnorm(9,22,2)),
                    Insulin_sd = c(rnorm(18,3,1)))
  
  
  p3 &lt;- ggplot(data = dat, aes(x = Time, y = Insulin, group = Trial))+
    geom_errorbar(aes(ymin = Insulin - Insulin_sd, ymax = Insulin + Insulin_sd,linetype = Trial), width = 4) +
    geom_line(aes(linetype = Trial)) +
    geom_point(aes(shape= Trial, fill = Trial), size = 2) +
    scale_shape_manual(values=c(21,24),guide = guide_legend(reverse = TRUE)) +
    scale_fill_manual(values=c("black","white"),guide = guide_legend(reverse = TRUE)) +
    scale_linetype_manual(values = c("solid","dashed"),guide = guide_legend(reverse = TRUE)) +
    scale_y_continuous(limits= c(0,35))
  </code></pre>
  
  <p>(sorry- not able to upload an image)</p>
  
  <p>Any help greatly appreciated.</p>