title: How to add custom ColorFunction in FillingStyle with Opacity
body: <p>I want to plot a series of lines with one half-space filled for each line. By setting opacity to something less than 1, I want to make the overlaps stand out. What I have looks something like this:</p>
  
  <pre><code>Plot[Table[x + a, {a, 0, 5}], {x, -1/2, 1/2}, 
   RegionFunction -&gt; Function[{x, y}, y &lt; 5],
   Filling -&gt; 5, FillingStyle -&gt; Directive[Opacity[0.25]]]
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/DYFrR.png" alt="enter image description here"></p>
  
  <p>This is fine. Now I want to also shade the colors for each half space in a particular way. Instead of the flat shading for each at present, say I want to shade it by the <code>y</code> value. I.e., if the flat shade color is blue, the shade of blue is scaled by <code>y</code> (0 most intense or 5 most intense doesn't matter). So at the first overlap, it automatically becomes <code>2y</code>, <code>3y</code> when two half-spaces overlay.</p>
  
  <p>How do I do this?</p>