title: add alpha legend to ggplot2 scatter plot
body: <p>My data looks something like this:</p>
  
  <pre><code>DF &lt;- data.frame(
    x = rnorm(100, mean = 0, sd = 1),
    y = rnorm(n = 100, mean = 1, sd = 1),
    color = sample(
      x = c('red', 'blue'), size = 100, replace = T, prob = c(.5,.5)
    ),
    alpha = runif(n = 100, min = 0.1, max = 1)
  )
  </code></pre>
  
  <p>I want to generate a scatter plot, and I want the only legend on the graph to be <code>alpha</code>, a continuous variable that can take values between 0.1 and 1.</p>
  
  <p>This is what I have right now:</p>
  
  <pre><code>DF %&gt;%
    ggplot(aes(
      x = x, y = y, color = color, size = 2, alpha = alpha
    )) +
    geom_point(show_guide = TRUE) + theme_bw() 
  </code></pre>
  
  <p>Is this possible?</p>