title: gnuplot: plot points with color based values in one string column and show string in legend
body: <p>I would like to plot results of classification and mark true classes. So, basically what I need is to assign a color for each point base on value in a string column. </p>
  
  <p>dataset looks like this:</p>
  
  <blockquote>
    <p>5.1   3.5 1.4 0.2 Iris-setosa</p>
  </blockquote>
  
  <p>I ended up with script following solution (thanks to the answer in here: <a href="https://stackoverflow.com/questions/8717805/vary-point-color-in-gnuplot-based-on-value-of-one-column">How to make points one color when a third column equals zero, and another color otherwise, in Gnuplot?</a>)</p>
  
  <pre><code>set palette model RGB defined (0 "red",1 "blue", 2 "green")
  plot 'iris.data' using 1:2:5 notitle with points pt 2 palette
  </code></pre>
  
  <p>in the original dataset I replaced string labels with numbers, because I don't know how to work with strings in gnuplot. Is there a way how to map string to colors?</p>
  
  <p>Currently the output looks like this:
  <img src="https://i.stack.imgur.com/vXJ76.png" alt="gnuplot coloring points"></p>
  
  <p>However I don't like the gradient palette because it doesn't make sense in this case. I would prefer normal legend with a single color and name of the class. Any idea how to do that?</p>