title: How do I make a boxplot with two categorical variables in R?
body: <p>I would like to make a boxplot that shows how time spent doing a behaviour(Alert) is affected by two variables (Period= Morning/Afternoon and Visitor Level= High/Low). </p>
  
  <pre><code>Alert ~ Period + Vis.Level
  </code></pre>
  
  <p>'Alert' is a set of 12 numbers that show the amount of time spent awake with the other two as the significant categorical variables. I have looked at other examples but none seem to fit this type of question. </p>
  
  <p>I know the graph I am looking for would have 4 boxplots on it... supposedly with </p>
  
  <ul>
  <li>PeriodMorning+Vis.LevelHigh</li>
  <li>PeriodMorning+Vis.LevelLow</li>
  <li>PeriodAfternoon+Vis.LevelHigh</li>
  <li>PeriodAfternoon+Vis.LevelLow</li>
  </ul>
  
  <p>on the x axis.</p>
  
  <p>Any help at all would be fantastic!</p>
  
  <pre><code>   Alert Vis.Level    Period
  1    0.0       Low   Morning
  2    1.0       Low   Morning
  3    0.0       Low   Morning
  4   11.5       Low Afternoon
  5    6.0       Low Afternoon
  6   11.5       Low Afternoon
  7    0.0      High   Morning
  8    0.0      High   Morning
  9    0.0      High   Morning
  10   0.0      High Afternoon
  11   2.5      High Afternoon
  12   7.5      High Afternoon
  </code></pre>