title: how to make a PCA plots as I posted here
body: <p>I <a href="https://stackoverflow.com/questions/34389869/how-to-draw-circle-in-pcoa-results">know how to use the PCA results to draw the circle</a>, but failed to draw the <code>x.lab</code> and the <code>y.lab</code> based on the plotting results from <code>s.class</code>. </p>
  
  <p>How to make a plot as I posted here?
  <img src="https://i.stack.imgur.com/Dhk4f.png" alt="two muppets">
   I would like to ask more about this. </p>
  
  <ol>
  <li><p>How to make the points bigger or smaller according to another integer variable? </p></li>
  <li><p>Can <code>ggplot2</code> draw the same circle as <code>s.class</code>? <a href="https://stackoverflow.com/questions/32259976/colouring-a-pca-plot-by-clusters-in-r">The previous answers</a> do not show how to draw circles.</p></li>
  </ol>