title: add horizontal line histogram gnuplot
body: <p>I would like to add a horizontal line in my histogram in gnuplot, is that possible?</p>
  
  <p>My histogram has on the x axis: alea1 alea 2 alea3 nalea1 nalea 2 nalea 3
  and the y axis goes from 0 to 25. </p>
  
  <p>At 22, I want to add a horizontal line that goes all the way across from one end to the other end of the histogram.</p>