title: Add text to ggplot
body: <p>(updated)
  I have ggplot like this, but then the x axis Date scaled:</p>
  
  <pre><code>g1 &lt;- ggplot(diamonds, aes(clarity, fill=cut)) + geom_bar()
  </code></pre>
  
  <p>Above two bars (lets say <code>VS2</code> and <code>IF</code>, but in my graph it is a date) I want to put a text label above the bars at height 13.000.</p>
  
  <p>I tried a lot of things, but this is what came closest:
  This is what I tried in my graph with the Date axis
  <code>g1 + geom_text(aes(as.Date("2014-10-05"), 13000), label="boat")</code></p>
  
  <p>but this adds only one to the graph and as soon as I try to extend it, for example with</p>
  
  <pre><code>g1 + geom_text(aes(c(as.Date("2014-10-05"),as.Date("2014-10-20")) , 13000), label=c("boat", "train"))
  </code></pre>
  
  <p>then I get the error:</p>
  
  <blockquote>
    <p>Error: Aesthetics must either be length one, or the same length as the
    dataProblems:c(as.Date("2014-10-05"), as.Date("2014-10-20"))</p>
  </blockquote>
  
  <p>I also tried to read the text and labels from a dataframe (<code>oefen</code>), where I used the same names as the original plot</p>
  
  <pre><code>g1 + geom_text(data=oefen, aes(x=newdat, y=Number, label=oefen$labs, fill=1))
  </code></pre>
  
  <p>I get the error</p>
  
  <blockquote>
    <p>Error: Continuous value supplied to discrete scale</p>
  </blockquote>
  
  <p>I tried many other solutions, but cannot find the answer. What am I missing?</p>