title: How do I add the mean value to a histogram in R?
body: <p>I would like to plot a histogram with mean (average) value on it (e.g. we could mark it with a blue and bold line).</p>
  
  <p>I tried to do it using <code>plot</code> command, but even if I set the parameter <code>add=TRUE</code>
  it didn't work.</p>