title: how to plot histogram and pdf together in r
body: <p>I want to superimpose the PDF of a fitted model following <code>gamma(lambda,k)</code> with a histogram.
  I write :</p>
  
  <pre><code>hist(pressure)
  curve(dgamma(x, lambda, k), add=TRUE, col="red")
  </code></pre>
  
  <p>but I am confused on what the value "x" is.
  Anyone help please?</p>