title: Set alpha and remove black outline of density plots in ggpairs
body: <p>Consider this example:</p>
  
  <pre><code>data(tips, package = "reshape")
  library(GGally)
  pm &lt;- ggpairs(tips, mapping = aes(color = sex), columns = c("total_bill", "time", "tip"))
  pm
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/lognV.png" rel="noreferrer"><img src="https://i.stack.imgur.com/lognV.png" alt="enter image description here"></a></p>
  
  <p>How do I make the density plots more transparent and remove the black lines?</p>
  
  <p>The <code>GGally</code> packages seems to have changed a lot recently and I cannot find a working solution</p>
  
  <p><strong>update</strong></p>
  
  <p>I found how to change the alpha with a custom function:</p>
  
  <pre><code>my_dens &lt;- function(data, mapping, ..., low = "#132B43", high = "#56B1F7") {
    ggplot(data = data, mapping=mapping) +
      geom_density(..., alpha=0.7) 
  }
  
  pm &lt;- ggpairs(tips, mapping = aes(color = sex), columns = c("total_bill", "time", "tip"),
                diag=list(continuous=my_dens))
  pm
  </code></pre>
  
  <p>but the black line still remains.</p>