title: What is the correct plot syntax for agent variables in NetLogo 5.1?
body: <p>I am trying to find the correct syntax for plotting agent variables in NetLogo 5.1.  </p>
  
  <p>I am attempting to plot the energy of breed SHARKS which varies according to their feeding and swimming.</p>
  
  <p>The example plot syntax from the Traffic Basic model is: <code>plot [speed] of sample-car</code></p>
  
  <p>However, if I therefore try: <code>plot [energy] of shark 1</code> then it throws this error : </p>
  
  <blockquote>
    <p>PLOT expected input to be a number but got the list [-7465] instead.</p>
  </blockquote>
  
  <p>If I use a <code>plot mean [energy] of shark 1</code> then it works (because it's the mean value of just one agent).</p>
  
  <p>Is the plot function expecting an operator applied to the agent variable?  If so, is there a list of accepted variables?  I feel the transition docs are lacking in this area.</p>
  
  <p>Thanks</p>
  
  <p>// EDIT:  Here's a simplified code of the shark/energy set-up.  I still get the same error, even with this.</p>
  
  <pre><code>breed [sharks shark]
  sharks-own [energy]
  
  to setup
   clear-all 
  
    create-sharks 1 [
       set energy 100
    ]
  
    reset-ticks
  end
  
  to go
    ask sharks [
      fd 1  
      set energy energy - 1  
    ]  
  
    tick
  end
  </code></pre>