title: Add horizontal line to ggplot() for specified interval of x axis
body: <p>I would like to add horizontal lines to an existing plot, but I would only like to plot the line for certain intervals of the x-axis.</p>
  
  <p>For example I would like to have a horizontal line at X=1:5 and y=50.</p>
  
  <p>I would use <code>existing_plot+geom_hline(yintercept = 50)</code></p>
  
  <p>Is it also possible to specify the x values somehow?</p>