title: how to add a line of best fit to ggplot within a group with facets
body: <p>I have a dataset comprising the following variables (fruit, prices, country, organic/non-organic, location). </p>
  
  <p>I would like a plot like the one here but with one thing added - a line of best fit that runs through the points for each grouping of organic/non-organic, location, and fruit. </p>
  
  <p>plot -> <a href="https://dl.dropboxusercontent.com/u/3803117/stackoverflow.jpeg" rel="nofollow noreferrer">https://dl.dropboxusercontent.com/u/3803117/stackoverflow.jpeg</a>
  <img src="https://i.stack.imgur.com/oCknl.jpg" alt="enter image description here">
  For example, in the "Organic, City" square, I would like 4 lines of best fits - one centered on Apples, Bananas, Cherries, Dates, etc.</p>
  
  <p>Here's the code I used to generate the plot.</p>
  
  <pre><code>p &lt;- ggplot(data,aes(factor(fruit),price)) + 
    geom_violin(aes(fill=Country,trim=FALSE)) + 
    geom_boxplot(aes(fill=Country),position=position_dodge(0.9),width=.1) + 
    geom_jitter(alpha=0.5) + 
    facet_wrap(organic~location) +
    xlab("Fruit") +
    ylab("Price") +
    labs(fill="Country")
  </code></pre>
  
  <p>Here's a sample dataset if it might help -> <a href="https://dl.dropboxusercontent.com/u/3803117/stackoverflow.csv" rel="nofollow noreferrer">https://dl.dropboxusercontent.com/u/3803117/stackoverflow.csv</a></p>
  
  <p>Thanks in advance so much for all the help!</p>