title: Plot difference in values of multiple variables in a dataframe
body: <p>I have dataframe <code>DF</code> that shows the performance of different students in two tests, <code>test1</code> and <code>test2</code>:</p>
  
  <pre><code>DF &lt;- data.frame(student = factor(c("S1", "S2", "S3", "S4", "S5")), 
                   test1 = factor(c(90, 77, 45, 67, 80)),
                   test2 = factor(c(70, 75, 55, 64, 80)))
  </code></pre>
  
  <p>I want to plot student performance over time, i.e. across the two tests in a plot that looks like this, but with <code>student</code> instead of <code>series</code> and tests instead of <code>time</code> (so (<code>test1</code> and test2`, rather than for example the numbers 2 and 4) on the x axis:<a href="https://i.stack.imgur.com/xGFF3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xGFF3.png" alt="enter image description here"></a></p>
  
  <p>How can I do it?</p>