title: Manually Set Scale of contour plot using geom_contour_filled
body: <p>I would like manually adjust the scales of two contour plots such that each have the same scale even though they contain different ranges of values in the z-direction.<br>
  For instance, lets say that I want to make contour plots of z1 and z2: </p>
  
  <pre><code>x = 1:15
  y = 1:15
  z1 = x %*% t(y)
  z2 = 50+1.5*(x %*% t(y))
  
  data &lt;- data.frame(
    x = as.vector(col(z1)),
    y = as.vector(row(z1)),
    z1 = as.vector(z1),
    z2 = as.vector(z2)
  )
  
  ggplot(data, aes(x, y, z = z1)) + 
    geom_contour_filled(bins = 8) 
  
  ggplot(data, aes(x, y, z = z2)) + 
    geom_contour_filled(bins = 8) 
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/5xQLg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5xQLg.png" alt="enter image description here"></a>
  <a href="https://i.stack.imgur.com/U72yR.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/U72yR.png" alt="enter image description here"></a></p>
  
  <p>Is there a way I can manually adjust the scale of each plot such that each contain the same number of levels (in this case bins = 8), the minimum is the same for both (in this case min(z1)), and the max is the same for both (max(z2))? </p>