title: R plot linear regression and standard deviation using abline
body: <p>I can plot a linear regression line using abline: </p>
  
  <pre><code>x= runif(10)
  y=runif(10)
  fit1 &lt;- lm(x ~ y )
  sd2 &lt;- sd(abs(fit1$residuals))*2
  
  plot(x,y)
  abline(fit1)
  </code></pre>
  
  <p>But how can I plot the standard deviation  lines using abline.
  I tried:</p>
  
  <pre><code>abline(fit1+sd2)
  abline(fit1-sd2)
  </code></pre>
  
  <p>but I get an error.
  Thank you for your help.</p>