title: ggplot() lines transparency
body: <p>How to change the transparency level of lines in <code>ggplot()</code> diagram (i.e. histogram, line plot, etc.)?</p>
  
  <p>For instance consider the code below:</p>
  
  <pre><code>data &lt;- data.frame(a=rnorm(100), b = rnorm(100,.5,1.2))
  data &lt;- melt(data)
  colnames(data) &lt;- c("Category", "Intensity")
  p &lt;- ggplot(data, aes(x=Intensity))
  p &lt;- p + geom_density(aes(color=Category), size=2, alpha=.4)
  print(p)
  </code></pre>
  
  <p>I expected the lines would be transparent (as <code>alpha=.4</code>), but they're not. </p>
  
  <p><img src="https://i.stack.imgur.com/m4VQr.png" alt="enter image description here"></p>