title: same area for all violins independent of facets in ggplot2
body: <p>I want to create a plot for three different factors where all violins have the same area. But using <code>facet_grid(. ~ C)</code> seems to force the violins within each facet (i.e. just those within a level of factor C) to have the same area. How can I overcome this?</p>
  
  <pre><code>library(ggplot2)
  
  d &lt;- data.frame(value = c(906, 1013, 1109, 876, 747, 759, 876, 1358, 739, 
                            1086, 807, 954, 1586, 762, 1353, 1221, 976, 1002, 
                            1129, 943, 1270, 1126, 853, 950, 677, 696, 681, 
                            615, 736, 595, 590, 618, 524, 1014, 515, 645, 860, 
                            874, 934, 728, 1078, 659, 1024, 786, 821, 541,
                            681, 744),
                  A = gl(2, 12, 48),
                  B = gl(2, 6, 48),
                  C = gl(2, 24))
  
  ggplot(d, aes(x = A, y = value, fill = B)) +
    geom_violin(trim = FALSE, scale = "area") +
    facet_grid(. ~ C)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/i7ciE.png" rel="noreferrer"><img src="https://i.stack.imgur.com/i7ciE.png" alt="enter image description here"></a></p>