title: Set title for arrow in gnuplot
body: <p>I've used the arrow to draw a vertical line and I would like to title it so it's shown in the key. Is there a way to do it? As far as I can tell for the manual, there's no <code>title</code> option in the syntaxis for <code>arrow</code>, but I'm sure there's a workaround. </p>
  
  <p>The only thing I think of is drawing the arrow with the same color as something outside the plot range and use its title, but it's rather clumsy.</p>
  
  <p>I'm using the terminal <code>pngcairo</code>, just in case it's relevant.</p>