title: R ggplot2: Labeling a horizontal line without associating the label with a series
body: <p>I'd like to label a horizontal line on a ggplot <em>with multiple series</em>, without associating the line with a series. <a href="https://stackoverflow.com/q/12876501/1840471">R ggplot2: Labelling a horizontal line on the y axis with a numeric value</a> asks about the single-series case, for which <code>geom_text</code> solves. However, <code>geom_text</code> associates the label with one of the series via color and legend.</p>
  
  <p>Consider the same example from that question, with another <code>color</code> column:</p>
  
  <pre><code>library(ggplot2)
  df &lt;- data.frame(y=1:10, x=1:10, col=c("a", "b"))  # Added col
  h &lt;- 7.1
  plot1 &lt;- ggplot(df, aes(x=x, y=y, color=col)) + geom_point()
  plot2 &lt;- plot1 + geom_hline(aes(yintercept=h))
  # Applying top answer https://stackoverflow.com/a/12876602/1840471
  plot2 + geom_text(aes(0, h, label=h, vjust=-1))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/YNekk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YNekk.png" alt="Result"></a></p>
  
  <p>How can I label the line without associating the label to one of the series?</p>