title: R ggplot multivariable facet
body: <p>I have df USArrests, that contains 3 crime statistics per US State and an additional UrbanPop variable. I'm trying to create faceted bar chart(s) that contain the 3 crime statistics of each state, with the y-axis being cases per 100,000 people at specific breaks (100, 200, 300). The top of the graph would read the state that it is from. </p>
  
  <p>My main issue is that i'm just not fully understanding the mapping/aes function and how that relates into pulling all 3 variables together in one graph. Thanks</p>
  
  <pre><code>library(ggplot2)
  
  USArrests$state_name &lt;- rownames(USArrests)
  
  p &lt;- ggplot() +
    layer(data = USArrests,
          stat = "identity",
          geom = "bar",
          mapping = aes(x = Assault, Rape, Murder),
            position = "identity") +
    scale_y_discrete(name = "Cases per 100,000 people",
                       breaks = c(100, 200, 300),
                       labels = c("100", "200", "300")) +
             facet_wrap(~ state_name)
  plot(p)
  </code></pre>