title: How to adjust figure settings in plotmatrix?
body: <p>Can I adjust the point size, alpha, font, and axis ticks in a plotmatrix?</p>
  
  <p>Here is an example:</p>
  
  <pre><code>library(ggplot2)
  plotmatrix(iris)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/ekDtL.jpg" alt="enter image description here"></p>
  
  <p>How can I:</p>
  
  <ul>
  <li>make the points twice as big</li>
  <li>set alpha = 0.5</li>
  <li>have no more than 5 ticks on each axis</li>
  <li>set font to 1/2 size?</li>
  </ul>
  
  <p>I have fiddled with the <code>mapping = aes()</code> argument to <code>plotmatrix</code> as well as <code>opts()</code> and adding layers such as <code>+ geom_point(alpha = 0.5, size = 14)</code>, but none of these seem to do anything. I have hacked a bit of a fix to the size by writing to a large pdf (<code>pdf(file = "foo.pdf", height = 10, width = 10)</code>), but this provides only a limited amount of control.    </p>