title: Flexible diagonal line plot in R
body: <p>How can we plot diagonal across (from bottom-left-hand corner to top
  right-hand corner),
  at any given coordinate ranges?</p>
  
  <p>For example</p>
  
  <pre><code>&gt; plot(c(-2,3), c(-1,5), type = "n", xlab="x", ylab="y", asp = 1)
  </code></pre>
  
  <p>or</p>
  
  <pre><code>&gt; plot(c(0,1000), c(0,334), type = "n", xlab="x", ylab="y", asp = 1)
  </code></pre>
  
  <p>I tried abline with the following but failed:</p>
  
  <pre><code>&gt; abline(0,1,col="red")
  </code></pre>