title: ggplot2 multiple sub groups of a bar chart
body: <p>I am trying to produce a bar graph that has multiple groupings of factors. An example from excel of what I am attempting to create, subgrouped by Variety and Irrigation treatment:</p>
  
  <p><img src="https://i.stack.imgur.com/lg0NU.png" alt="enter image description here"></p>
  
  <p>I know I could produce multiple graphs using <code>facet_wrap()</code>, but I would like to produce multiple graphs for this same type of data for multiple years of similar data. An example of the data I used in this example:</p>
  
  <pre><code>Year        Trt Variety    geno yield   SE
  2010-2011   Irr Variety.2   1   6807    647
  2010-2011   Irr Variety.2   2   5901    761
  2010-2011   Irr Variety.1   1   6330    731
  2010-2011   Irr Variety.1   2   5090    421
  2010-2011   Dry Variety.2   1   3953    643
  2010-2011   Dry Variety.2   2   3438    683
  2010-2011   Dry Variety.1   1   3815    605
  2010-2011   Dry Variety.1   2   3326    584
  </code></pre>
  
  <p>Is there a way to create multiple groupings in ggplot2? I have searched for quite some time and have yet to see an example of something like the example graph above.</p>
  
  <p>Thanks for any help you may have!</p>