title: How come geom_hline and geom_text behave differently in ggplot2?
body: <p>Using <code>geom_text</code> without <code>aes</code> will render a text multiple times (actually, <code>nrow(iris)</code> times):</p>
  
  <pre><code>ggplot(iris) +
    geom_point(aes(x=Petal.Length, y=Petal.Width)) +
    geom_text(label='afasd', x=1, y=0.5)
  </code></pre>
  
  <p>In fact, this is how most <code>geom_xxx</code> layer function works, unless a <code>data</code> parameter is specified. However, <code>geom_hline</code> doesn't behave quite the same:</p>
  
  <pre><code>ggplot(iris) +
    geom_point(aes(x=Petal.Length, y=Petal.Width)) +
    geom_hline(yintercept=1)
  </code></pre>
  
  <p>This draws the <code>hline</code> only once.</p>
  
  <p>Is it supposed to be this way, or is it a mistake?</p>