title: How to set the opacity for a plot?
body: <p>I have some data to be plotted in one figure. Noise data is ruining other data. How can I change the transparency level of a given data? In my case, I'm using <code>hold all</code> command for plotting several data. One of the solution is to change the <code>LineWidth</code> but I couldn't find a way for transparency option. I've tried <code>alpha</code> as follows </p>
  
  <p><code>plot( noise_x, 'k', 'LineWidth', 1, 'alpha', 0.2)</code></p>
  
  <p>but with no luck. </p>