title: Why ggplot2 could not split my plots using facet_grid?
body: <p>Here is my data and code:</p>
  
  <pre><code>a &lt;- data.frame(Original = c("Full", "Full", "T0", "T0"),
              Classified = c("Full", "T0", "Full", "T0"),
              Value = c(0.69, 0.31, 0.32, 0.68),
              Text = c("69%", "31%", "32%", "68%"))
  
  
  ggplot(data = a, aes(x=Classified, y=Original, fill=Value)) + 
    geom_tile()+
    geom_text (aes (label=Text), size = 15)+
    scale_fill_gradient(low="light blue", high="purple")+
    facet_grid (Original~Classified, space = "free", switch = "y")+
    theme_bw()+
    ggtitle ("ADS")+
    theme (panel.background = element_blank())+
    theme (plot.background = element_blank())+
    theme (panel.grid.major = element_line(colour = "white"))+
    ylab ("Percentage of Tokens")+
    theme (strip.text = element_text(size = 23))+
    theme (axis.title = element_text (size = 25))+
    theme(plot.title = element_text(size=25))+
    theme (legend.position = "none")+
    theme (axis.title.x = element_blank())
  </code></pre>
  
  <p>Why it generate sth like the following? It should fully fill the four plots in the four panels.
  <a href="https://i.stack.imgur.com/SBQQM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/SBQQM.png" alt="enter image description here"></a></p>