title: ggplot2 offset scatterplot points
body: <p>I have two sets of points with error bars.  I would like to offset the second so it's displayed slightly down from the first set, so that it doesn't obscure the original.</p>
  
  <p>Here is a mock data set:</p>
  
  <pre><code>x=runif(4,-2,2)
  y=c("A","B","C","D")
  upper=x+2
  lower=x-2
  x_1=runif(4,-1,3)
  upper_1=x_1+1
  lower_1=x_1-2
  </code></pre>
  
  <p>Here is the code that I used to produce the plot:</p>
  
  <pre><code>qplot(x,y)+
    geom_point(size=6)+
    geom_errorbarh(aes(xmax=upper,xmin=lower),size=1)+
    geom_point(aes(x_1,y),size=6,pch=8,vjust=-1,col="grey40")+
    geom_errorbarh(aes(xmax=upper_1,xmin=lower_1),size=1,col="grey40")
  </code></pre>
  
  <p>And here is the plot:</p>
  
  <p><img src="https://i.stack.imgur.com/UFLtD.png" alt="scatterplot"></p>
  
  <p>I would like the grey asterisks and associated errors bars to be plotted a hair below the black circles and associated error bars.  I would transform the data set, but the Y-axis is categorical variables.</p>