title: how to draw guide lines on a gnuplot generated cdf?
body: <p>At work have a set of floating point values that I sort and compute a CDF for and plot within gnuplot.  I'd like to draw a line showing where the 80% and 90% thresholds of the CDF are, i.e. a line coming in from the left @ the 0.8 y tic mark, touching the graph and then dropping down to whatever that value might be.  This is to help guide the viewers eye.  </p>
  
  <p>The data is generated automatically and I make multiple plots so I don't want to have to hand craft these lines each time.</p>
  
  <p>It's trivial to draw a horizontal arrow going completely across the plot at the 0.8 and 0.9 y-value points, but I don't understand how to determine where the vertical line should be drawn.
  Here is a q/a wrt drawing arrows: <a href="https://stackoverflow.com/questions/4499998/gnuplot-vertical-lines-at-specific-positions/8562193#8562193">Gnuplot: Vertical lines at specific positions</a>, but the positions are known a priori.</p>
  
  <p>Here is some sample data (my work machine is not internet accessible so sharing is hard)</p>
  
  <pre><code>  X                Y
   5.0   |         0.143
   8.0   |         0.288
  16.0   |         0.429
  25.0   |         0.714
  39.0   |         0.857
  47.0   |         1.000
  </code></pre>
  
  <p>Any ideas?</p>