title: Plotting multiple independent variables with R
body: <p>Say I have a dataframe constructed like this:</p>
  
  <pre><code>x1 &lt;- c(1,1,1,1,2,2,2,2)
  x2 &lt;- c(1,1,2,2,1,1,2,2)
  x3 &lt;- c(1,2,1,2,1,2,1,2)
  y &lt;- c(1:8)
  d &lt;- data.frame(x1,x2,x3,y)
  </code></pre>
  
  <p>To help analyze the data, I can use <code>plot(d)</code> to generate 6 graphs to see the effects of x1, x2, and x3 on y. Is there a way to color points differently based on the value of x1 with their symbol based of the value of x2 and their size based off of the value of x3? It doesn't need to be these specific properties. But when I try to add color or change the character with</p>
  
  <p><code>plot(d, col=c("red", "blue"), pch=c(19, 21))</code></p>
  
  <p>Only x3 is affected. Is there a way to specify which points receive which properties? Or maybe what I'm trying to accomplish is easier with ggplot2?</p>