title: CorePlot using Rubymotion
body: <p>I'm playing around with CorePlot using rubymotion.</p>
  
  <p>Can anyone help me translate this objective c method call to rubymotion syntax?</p>
  
  <pre><code>overlayGradient = [overlayGradient addColorStop:[[CPTColor blackColor] colorWithAlphaComponent:0.0] atPosition:0.9];
  </code></pre>