title: One shared legend for a cowplot grid in R
body: <p>I just built a grid with package <code>cowplot</code> (to label the plots from A-D). The plots are made with package <code>ggplot2</code>:</p>
  
  <pre><code>pfour&lt;-ggplot(four, aes(x=Concentration, y=Percentage, fill=Phenotype)) + 
   geom_bar(stat='identity',color='black') +
   scale_fill_grey(start = .4, end = .9) + 
   theme_bw()+ylab("Distribution") + 
   xlab("Contentration [mg/ml]") + 
   ggtitle("96 hpf") +
   theme(legend.title = element_text(colour="black", size=10, face="bold")) +
   theme(legend.background = element_rect(fill="white",
                                          size=0.5, linetype="solid", 
                                          colour ="black")) +
   scale_x_discrete(limits=c('uninjected','control','0.002', '0.02', '0.2'),
                    labels=c('uninjected\n(n=251)',
                             'control\n(n=248)', 
                             '0.002\n(n=205)', 
                             '0.02\n(n=222)', 
                             '0.2\n(n=203)'))
  </code></pre>
  
  <p>the data looks like that (4 different tables with slightly different percentages but the same principle):</p>
  
  <pre><code>Concentration,Percentage,Phenotype
  uninjected,0.996015936,0
  uninjected,0,1
  uninjected,0.003984064,2
  uninjected,0,3
  uninjected,0,4
  control,0.995967742,0
  control,0.004032258,1
  control,0,2
  control,0,3
  control,0,4
  0.002,0.985365854,0
  0.002,0.004878049,1
  0.002,0.004878049,2
  0.002,0,3
  0.002,0.004878049,4
  0.02,0.981981982,0
  0.02,0.004504505,1
  0.02,0.004504505,2
  0.02,0.004504505,3
  0.02,0.004504505,4
  0.2,0.985221675,0
  0.2,0.004926108,1
  0.2,0,2
  </code></pre>
  
  <p>and it looks like that:</p>
  
  <p><a href="https://i.stack.imgur.com/kL4vQ.png" rel="noreferrer"><img src="https://i.stack.imgur.com/kL4vQ.png" alt="plot"></a></p>
  
  <p>the code for that is :</p>
  
  <pre><code>plot_grid(ponezoom, ptwozoom,pthreezoom,pfourzoom, align='h', labels=c('A', 'B','C','D'))
  </code></pre>
  
  <p>Now I was wondering if it is possible to get <strong>one single shared legend for all four plots</strong> as it steals a lot of plotspace to have it 4 times. I appreciate any help.</p>