title: Arrange 8 plots and a legend in a grid
body: <p>I have 8 plots and one legend that I would like to arrange in a 3x3 grid, so that the axes all align.</p>
  
  <p>It works perfectly fine when I am not including the legend to my grid, but as soon as I have added the legend, my plots get shifted and are no longer aligned.</p>
  
  <pre><code>plot_grid(SiO2TiO2, SiO2Al2O3, SiO2FeO, SiO2MgO, SiO2CaO, SiO2Na2O, SiO2K2O, SiO2P2O5, align=c("hv"),nrow = 3, ncol = 3)
  </code></pre>
  
  <p>Plots aligned in a 3x3 grid without legend:
  <img src="https://i.stack.imgur.com/Kplkz.png" alt=""></p>
  
  <pre><code>plot_grid(SiO2TiO2, SiO2Al2O3, SiO2FeO, SiO2MgO, SiO2CaO, SiO2Na2O, SiO2K2O, SiO2P2O5, legend, align=c("hv"),nrow = 3, ncol = 3)
  </code></pre>
  
  <blockquote>
    <p>Warning messages:</p>
    
    <p>1: In align_plots(plotlist = plots, align = align) :
      Graphs cannot be vertically aligned. Placing graphs unaligned.</p>
    
    <p>2: In align_plots(plotlist = plots, align = align) :
      Graphs cannot be horizontally aligned. Placing graphs unaligned.</p>
  </blockquote>
  
  <p>Plots not aligned in a 3x3 grid with legend:
  <img src="https://i.stack.imgur.com/IN1V6.png" alt=""></p>
  
  <p>Anyone know how to align the legend with the plots?
  Thanks in advance!</p>