title: Overlaying two ggplot facet_wrap histograms
body: <p>So I have two histogram plots I can do one at a time. The result using the following code gives a 2 row x 3 col facet plot for six different histograms:</p>
  
  <pre><code>ggplot(data) +
      aes(x=values) +
      geom_histogram(binwidth=2, fill='blue', alpha=0.3, color="black", aes(y=(..count..)*100/(sum(..count..)/6))) +
      facet_wrap(~ model_f, ncol = 3)
  </code></pre>
  
  <p>Here the <code>aes(y...)</code> just gives the percentage instead of counts.</p>
  
  <p>As stated, I have two of this 6 facet_wrap plot, which I now which to combine to show that one is more shifted than the other. 
  In addition, the data size is not the same, so for one I have:</p>
  
  <pre><code># A tibble: 5,988 x 5
     values ID   structure   model   model_f
     &lt;dbl&gt; &lt;chr&gt;     &lt;chr&gt;   &lt;chr&gt;    &lt;fctr&gt;
   1     6     1    bone       qua   Model I
   2     7     1    bone       liu  Model II
   3    20     1    bone       dav Model III
   4     3     1    bone       ema  Model IV
   5     3     1    bone       tho   Model V
   6     4     1    bone      ranc  Model VI
   7     3     2    bone       qua   Model I
   8     5     2    bone       liu  Model II
   9    18     2    bone       dav Model III
  10     2     2    bone       ema  Model IV
  # ... with 5,978 more rows
  </code></pre>
  
  <p>And the other:</p>
  
  <pre><code># A tibble: 954 x 5
      values  ID structure   model   model_f
     &lt;dbl&gt;  &lt;chr&gt;     &lt;chr&gt;   &lt;chr&gt;    &lt;fctr&gt;
   1     9     01    bone       qua   Model I
   2     8     01    bone       liu  Model II
   3    22     01    bone       dav Model III
   4     6     01    bone       ema  Model IV
   5     5     01    bone       tho   Model V
   6     9     01    bone       ran  Model VI
   7    12     02    bone       qua   Model I
   8    11     02    bone       liu  Model II
   9    24     02    bone       dav Model III
  10     9     02    bone       ema  Model IV
  # ... with 944 more rows
  </code></pre>
  
  <p>So they are not the same size, the ID's are not the same (data not related), but still, I wish to merge the histograms in order to see the difference between the data.</p>
  
  <p>I thought this might do the trick:</p>
  
  <pre><code>ggplot() +
      geom_histogram(data=data1, aes(x=values), binwidth=1, fill='blue', alpha=0.3, color="black", aes(y=(..count..)*100/(sum(..count..)/6))) +
      geom_histogram(data=data2, aes(x=values), binwidth=1, fill='blue', alpha=0.3, color="black", aes(y=(..count..)*100/(sum(..count..)/6))) +
      facet_wrap(~ model_f, ncol = 3)
  </code></pre>
  
  <p>However, that didn't do much.</p>
  
  <p>So now I'm stuck. Is this possible to do, or...?</p>