title: color points on plot depending of a value of a column
body: <p>I have a dataset with 2 columns. With <code>plot(ds$values)</code> I get all the points in black.</p>
  
  <p>Now consider that I have one second column that gives me categories as strings. Notice that I don't know the categories names. How do I plot each category with a different colour. </p>
  
  <p>A simple: <code>plot(ds$values, col=someFn(ds$categories))</code>?</p>
  
  <p>Adding an index would be good but not required.</p>