title: Add a title to C3.js chart legend
body: <p>I am using c3js and I'm still new to this chart framework. I need to set a title to the series legend of my chart. How can I do this? Do I need to use D3 to implement this? 
  <img src="https://i.stack.imgur.com/3UcJI.png" alt="I need something like this"></p>
  
  <p>Thanks in advance.</p>