title: Plotting arrows with gnuplot
body: <p>I have data generated in a simulation. The generated data file looks something like this:</p>
  
  <pre><code>1990/01/01 99
  1990/01/02 92.7
  1990/01/03 100.3
  1990/01/04 44.2
  1990/01/05 71.23
  ...
  2100/01/01 98.25
  </code></pre>
  
  <p>I can create a chart (trivially), by simply issuing the (long versioned) command:</p>
  
  <pre><code>plot "simulation.dat" using 1:2 with line
  </code></pre>
  
  <p>I want to add a third column which will add arrow information. The encoding for the third column would be as follows:</p>
  
  <ul>
  <li>0 => no arrow to be drawn for that x axis value</li>
  <li>1 => an UPWARD pointing arrow to be drawn for the x axis value</li>
  <li>2 => a DOWNWARD arrow to be drawn for the x axis value</li>
  </ul>
  
  <p>I am just starting to learn gnuplot, and will appreciate help in how I can use gnuplot to create the arrows on the first plot?</p>