title: discrete and continuous scales on same axis in facet_grid
body: <p>I want to use <code>facet_grid</code> in <code>ggplot2</code> to produce two graphs with continuous and discrete scales on the same axis - essentially my vector <code>fac</code> has a numeric part and a factor part.</p>
  
  <pre><code>toy=data.frame(type=xc("f f f f i i i i"),fac=c(letters[1:4],1:3,40))
  ggplot(toy,aes(x=ifelse(type=="f",fac,as.numeric((fac)))))+
     geom_bar()+facet_grid(.~type,scales="free")
  </code></pre>
  
  <p>But it doesn't work because <code>ggplot</code> forces both scales to be discrete.
   <img src="https://i.stack.imgur.com/Nn7rL.png" alt="enter image description here"></p>
  
  <p>As you can see from the equal spacing of 1,2,3,40 on the right-hand plot. </p>