title: Plot two variables on y axis
body: <p>I've been trying to plot two variables on one y axis. Here is my code:</p>
  
  <pre><code>set.seed(20)
  x1=rnorm(100)
  x2=rnorm(100)
  x3=rnorm(100)
  t=data.frame(a=x1,b=x1+x2,c=x1+x2+x3)
  plot(t$a,t$b,xlab="",ylab="",pch=0,col="red")
  par(new=TRUE)
  plot(t$a,t$c,xlab="a",ylab="b (blue) and c (red)",
  col="blue",abline(c   (0,0),c (1,1),lty=5,col=320))
  </code></pre>
  
  <p>Because the two y axises use different scales there're two marks on the y axis. How do I modify my code? You help is very appreciated!</p>