title: How to draw a level line in a filled contour plot and label it?
body: <p>I have the following contour plot</p>
  
  <pre><code>x &lt;- c(0,25,50,75,100)
  y &lt;- c(0,10,20)
  z &lt;- matrix(c(12,12,13,12,5,12,5,5,5,12,5,12,13,14,15), nrow = 5, ncol = 3, byrow = TRUE)
  A &lt;- matrix(seq(0, 100, by = 25), nrow = 3, ncol = 5, byrow = TRUE) #As x
  B &lt;- matrix(seq(0,20, by = 10), nrow = 3, ncol = 5) #As y
  filled.contour(x,y,z, color=terrain.colors,#
    plot.axes = { axis(1); axis(2); points(A,B)})
  </code></pre>
  
  <p>How can I draw a level line around the level with value 5 and label it and obtain something like:</p>
  
  <p><a href="https://i.stack.imgur.com/Cu7Yp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Cu7Yp.png" alt="enter image description here"></a></p>