title: Group data and plot multiple lines
body: <p>I'd like to plot multiple lines in R for this dataset: (x = Year, y = Value)</p>
  
  <pre><code>School_ID   Year    Value
  A           1998    5
  B           1998    10
  C           1999    15
  A           2000    7
  B           2005    15
  </code></pre>
  
  <p>Each school has data for different years. I'd like to have one line for each school.</p>