title: R error "could not find function 'multiplot' " using Cookbook example
body: <p>Would like to plot two ggplots on one page. Took the example from <a href="http://www.cookbook-r.com/Graphs/Multiple_graphs_on_one_page_%28ggplot2%29/" rel="nofollow">Cookbook for R</a> and it doesn't work. The error is <code>could not find function "multiplot"</code>.</p>
  
  <p>However ggplots are plotable, also I reinstalled R, ggplot2, restarted, etc..
  Am I doing something wrong?</p>
  
  <pre><code>library(ggplot2)
  
  # This example uses the ChickWeight dataset, which comes with ggplot2
  # First plot
  p1 &lt;- 
    ggplot(ChickWeight, aes(x=Time, y=weight, colour=Diet, group=Chick)) +
    geom_line() +
    ggtitle("Growth curve for individual chicks")
  
  # Second plot
  p2 &lt;- 
    ggplot(ChickWeight, aes(x=Time, y=weight, colour=Diet)) +
    geom_point(alpha=.3) +
    geom_smooth(alpha=.2, size=1) +
    ggtitle("Fitted growth curve per diet")
  
  # Third plot
  p3 &lt;- 
    ggplot(subset(ChickWeight, Time==21), aes(x=weight, colour=Diet)) +
    geom_density() +
    ggtitle("Final weight, by diet")
  
  # Fourth plot
  p4 &lt;- 
    ggplot(subset(ChickWeight, Time==21), aes(x=weight, fill=Diet)) +
    geom_histogram(colour="black", binwidth=50) +
    facet_grid(Diet ~ .) +
    ggtitle("Final weight, by diet") +
    theme(legend.position="none")        # No legend (redundant in this graph)    
  
  multiplot(p1, p2, p3, p4, cols=2)
  </code></pre>