title: Map size to more than one variable with geom_point in ggplot2
body: <p>Is there any option to use two variables in parameter <code>size</code> in <code>ggplot</code> function?
  E.g.</p>
  
  <pre><code>ggplot(mtcars, aes(qsec,drat, size = cyl))+
     geom_point()
  </code></pre>
  
  <p>Obviously it works and size of points depends on <code>cyl</code> variable. But is it any option to add another <code>size</code> variable which can use <code>alpha</code> option e.g. <code>variable = mpg</code>. How to do it?</p>
  
  <p>I did it just for three dots as an example in Ilustrator.
  <a href="https://i.stack.imgur.com/PAJLl.png" rel="noreferrer"><img src="https://i.stack.imgur.com/PAJLl.png" alt="enter image description here"></a></p>
  
  <p>Thanks,</p>
  
  <p><strong>Update</strong></p>
  
  <p>Thanks to @lukeA following lines of code works:</p>
  
  <pre><code>ggplot() + 
      geom_point(data = mtcars, aes(qsec,drat, size = cyl)) + 
      geom_point(data = mtcars, aes(qsec,drat, size = mpg), alpha = .1, colour = "red")
  </code></pre>
  
  <p>But when I want to set the size of each variable separately, it is not possible. When there is one <code>size</code> variable normally I use <code>scale_size_contiunous</code> but with two <code>size</code> variables it doesn't work. I know that using <code>scale_size_continuous</code> two times doesn't change anything. Maybe it is not possible at all, but maybe somebody find the solution.</p>