title: ggplot: Generate facet grid plot with multiple series
body: <p>I have following data frame:</p>
  
  <pre><code>  Quarter        x        y         p         q
  1  2001   8.714392 8.714621 3.3648435 3.3140090
  2  2002   8.671171 8.671064 0.9282508 0.9034387
  3  2003   8.688478 8.697413 6.2295996 8.4379698
  4  2004   8.685339 8.686349 3.7520135 3.5278024
  </code></pre>
  
  <p>My goal is to generate a facet plot where x and y column in one plot in the facet and p,q together in another plot instead of 4 facets. </p>
  
  <p>If I do following: </p>
  
  <pre><code>x.df.melt &lt;- melt(x.df[,c('Quarter','x','y','p','q')],id.vars=1)
  ggplot(x.df.melt, aes(Quarter, value, col=variable, group=1)) + geom_line()+
    facet_grid(variable~., scale='free_y') +
    scale_color_discrete(breaks=c('x','y','p','q'))
  </code></pre>
  
  <p>I all the four series in 4 different facets but how do I combine x,y to be one while p,q to be in another together. Preferable no legends.</p>
  
  <p><a href="https://i.stack.imgur.com/ZABZG.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ZABZG.png" alt="enter image description here"></a></p>