title: Is there a way to plot with 2 y values and one x
body: <p>I want to make a plot which has two y values and I can't seem to be able to find any way to make it work I'd imagne the code would look something like this:</p>
  
  <pre><code>x1 = c(1,2,3,4,5)
  y1 = c(10,13,15,17,11)
  y2 = c(5,7,8,3,5)
  
  plot(x1, y1, y2)
  </code></pre>
  
  <p>but I get the error:</p>
  
  <pre><code>Error in plot.xy(xy, type, ...) : invalid plot type
  </code></pre>