title: Adjust Transparency (alpha) of stat_smooth lines, not just transparency of Confidence Interval
body: <p>Using ggplot2's stat_smooth(), I'm curious how one might adjust the transparency of the generated regression line. Using geom_points() or geom_line(), one normally sets a value for 'alpha', indicating the percent transparency. However, with stat_smooth(), alpha sets the transparency of the confidence interval (in my sample below, turned off - se=FALSE).</p>
  
  <p>I cannot seem to find a way to make the regression line(s) a lower transparency than 1. </p>
  
  <p>Your advice would be wonderful.</p>
  
  <p><strong>Sample Code</strong></p>
  
  <pre><code> library(reshape2)
   df &lt;- data.frame(x = 1:300)
   df$y1 &lt;-  0.5*(1/df$x + 0.1*(df$x-1)/df$x + rnorm(300,0,0.015))
   df$y2 &lt;-  0.5*(1/df$x + 0.3*(df$x-1)/df$x + rnorm(300,0,0.015))
   df$y3 &lt;-  0.5*(1/df$x + 0.6*(df$x-1)/df$x + rnorm(300,0,0.015))
   df &lt;- melt(df, id = 1)
  
   ggplot(df, aes(x=x, y=value, color=variable)) +
     geom_point(size=2) +
     stat_smooth(method = "lm", formula = y ~ 0 + I(1/x) + I((x-1)/x),
                 se = FALSE,
                 size = 1.5,
                 alpha = 0.5)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/v5lEK.png" alt="enter image description here"></p>