title: How to add different lines for facets
body: <p>I have data where I look at the difference in growth between a monoculture and a mixed culture for two different species. Additionally, I made a graph to make my data clear. </p>
  
  <p>I want a barplot with error bars, the whole dataset is of course bigger, but for this graph this is the <code>data.frame</code> with the means for the barplot. </p>
  
  <pre><code>plant           species     means
  Mixed culture   Elytrigia   0.886625
  Monoculture     Elytrigia   1.022667
  Monoculture     Festuca     0.314375
  Mixed culture   Festuca     0.078125
  </code></pre>
  
  <p>With this data I made a graph in <code>ggplot2</code>, where <code>plant</code> is on the x-axis and <code>means</code> on the y-axis, and I used a facet to divide the species. </p>
  
  <p>This is my code: </p>
  
  <pre><code>    limits &lt;- aes(ymax = meansS$means + eS$se, ymin=meansS$means - eS$se)
      dodge &lt;- position_dodge(width=0.9)
  
      myplot &lt;- ggplot(data=meansS, aes(x=plant, y=means, fill=plant)) + facet_grid(. ~ species) 
      myplot &lt;- myplot + geom_bar(position=dodge) + geom_errorbar(limits, position=dodge, width=0.25)
      myplot &lt;- myplot + scale_fill_manual(values=c("#6495ED","#FF7F50"))
      myplot &lt;- myplot + labs(x = "Plant treatment", y = "Shoot biomass (gr)")  
      myplot &lt;- myplot + opts(title="Plant competition")
      myplot &lt;- myplot + opts(legend.position = "none")
      myplot &lt;- myplot + opts(panel.grid.minor=theme_blank(), panel.grid.major=theme_blank())
  </code></pre>
  
  <p>So far it is fine. However, I want to add two different horizontal lines in the two facets. For that, I used this code: </p>
  
  <pre><code>    hline.data &lt;- data.frame(z = c(0.511,0.157), species = c("Elytrigia","Festuca")) 
      myplot &lt;- myplot + geom_hline(aes(yintercept = z), hline.data)
  </code></pre>
  
  <p>However if I do that, I get a plot were there are two extra facets, where the two horizontal lines are plotted. Instead, I want the horizontal lines to be plotted in the facets with the bars, not to make two new facets. Anyone a idea how to solve this. </p>
  
  <p>I think it makes it clearer if I put the graph I create now: </p>
  
  <p><img src="https://i.stack.imgur.com/pMWs2.png" alt="enter image description here"> </p>