title: Make bars in JFreeChart semi-transparent?
body: <p>I used JFreeChart to create a <code>XYBarChart</code>, and I want to know if it is possible to make the green bars semi-transparent if there is another set of bars hiding behind it. In other words, perhaps changing the opacity of only the green bars that are on top of the blue ones?</p>
  
  <p><img src="https://i.stack.imgur.com/Tx2et.png" alt="enter image description here"></p>