title: How to overlay scatterplots in R?
body: <p>If I have three sets of data :</p>
  
  <pre><code>a1= rnorm(10)
  a2= rnorm(10)
  a3= rnorm(10)
  </code></pre>
  
  <p>rather than looking at these side by side using:</p>
  
  <pre><code>par(mfrow=c(1,3))
  plot(a1)
  plot(a2)
  plot(a3)
  </code></pre>
  
  <p>How do I get all these points on the same plot?</p>