title: Add legend to a plot with R
body: <p>I create 4 line in one plot using R .
  Here the code</p>
  
  <pre><code># Define 2 vectors
  cars &lt;- c(123.07, 110.51, 96.14, 98.71, 101.3)
  trucks &lt;- c(110.31, 89.91, 89.81, 89.31, 93.4, 95.81)
  cars1 &lt;- c(123.227, 110.221, 93.14, 98.22, 122.3)
  trucks1 &lt;- c(120.31, 89.91, 89.81, 89.31, 93.4, 95.81)
  # Graph cars using a y axis that ranges from 0 to 12
  plot(cars, type="o", col="blue", ylim=c(80,130))
  
  # Graph trucks with red dashed line and square points
  lines(trucks, type="o", pch=22, lty=2, col="red")
  lines(cars1, type="o", col="yellow", ylim=c(80,130))
  
  # Graph trucks with red dashed line and square points
  lines(trucks1, type="o", pch=22, lty=2, col="green")
  # Create a title with a red, bold/italic font
  title(main="Autos", col.main="red", font.main=4)
  </code></pre>
  
  <p>I would like to know how can I add the legend for each plot (line) in the plot.</p>
  
  <p>Thank you</p>