title: Graphviz: give color to lines with rainbow effect
body: <p>I have dataframe and I build graph using <code>graphviz</code></p>
  
  <pre><code>for id_key, group in df.groupby('ID'):
  f = Digraph('finite_state_machine', filename='fsm.gv', encoding='utf-8')
  f.body.extend(['rankdir=LR', 'size="5,5"'])
  f.attr('node', shape='box')
  for i in range(len(group)-1):
      f.edge(str(group['category'].iloc[i]), str(group['category'].iloc[i+1]),
             label=str(group['search_term'].iloc[i+1]))
  f.render(filename=str(id_key))
  </code></pre>
  
  <p>and get this result 
  <a href="https://i.stack.imgur.com/2s2WN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2s2WN.png" alt="image"></a>. How can I change lines color: first arrow - red, second - orange, third - yellow, etc?</p>