title: How to add labels in a contour plot using ggplot2?
body: <p>I'm doing a contour plot with <code>ggplot2</code> and I'm trying to add the labels like <a href="https://eliocamp.github.io/metR/reference/geom_text_contour.html" rel="nofollow noreferrer">this tutorial</a>. However, always appears the following error:</p>
  
  <p><code>Error in geom_text_contour(aes(z = z)) : could not find function "geom_text_contour"</code></p>
  
  <pre><code>library(ggplot2)
  
  x &lt;- c(0,0,0,1,1,1,2,2,2,3,3,3)
  y &lt;- c(0,1,2,0,1,2,0,1,2,0,1,2)
  z &lt;- seq(0,11,1)
  
  df &lt;- data.frame(x,y,z)
  
  ggplot(df, aes(x, y, z = z)) +
    geom_tile(aes(fill = z)) +
    geom_contour(colour = "white") +
    geom_text_contour(aes(z = z))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/XUH14.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/XUH14.png" alt="enter image description here"></a></p>