title: Remove thing from gnuplot's legend
body: <p>I have to plot three implicit functions using gnuplot, I use this:</p>
  
  <pre><code>set contour
  set cntrparam levels discrete 0
  set view map
  unset surface
  set isosamples 1000,1000
  set xrange [-5:7]
  set yrange [-15:15]
  set xlabel "x"
  set ylabel "y"
  splot y**2-x**3+15*x-13 t "t1", y**2-x**3+15*x-sqrt(4.*15.**3/27.) t "singular", y**2-x**3+15*x-30 t "t2", y**2-x**3+15*x-13 t "t3"
  </code></pre>
  
  <p>And the output is this:
  <img src="https://i.stack.imgur.com/OGDWB.png" alt="enter image description here"></p>
  
  <p>The program is writing the 0 of the level of the surface in the legend but I just want the <code>title</code> parameter passed to the splot command. As the three surfaces are actually the same at a different height, I could change <code>set cntrparam...</code> line to draw the three of them, but what I want to do is to remove the numbers and make it write just text. How can I do that?</p>