title: How to plot a normal distribution by labeling specific parts of the x-axis?
body: <p>I am using the following code to create a standard normal distribution in R:</p>
  
  <pre><code>x &lt;- seq(-4, 4, length=200)
  y &lt;- dnorm(x, mean=0, sd=1)
  plot(x, y, type="l", lwd=2)
  </code></pre>
  
  <p>I need the x-axis to be labeled at the mean and at points three standard deviations above and below the mean. How can I add these labels?</p>