title: How to remove the grey background from a facet_grid() (ggplot2)
body: <p>I have the following plot:</p>
  
  <p><img src="https://i.stack.imgur.com/x375i.png" alt="enter image description here"></p>
  
  <p>obtained by executing:</p>
  
  <pre><code>ggplot() + 
    stat_summary( data = test_data1, aes(x=x, y=(1-value) , colour = as.factor(1) , lty = as.factor(1) ) , fun.y=mean, geom="line" ,  size=1 ) +
    stat_summary( data = test_data1, aes(x=x, y=(1-value) , colour = as.factor(1), shape=as.factor(1) ) , fun.y=mean, geom="point" ,  size=3, pch=21, fill="white" ) +
    stat_summary( data = test_data2, aes(x=x, y=(1-value) , colour = as.factor(2), lty = as.factor(2) ) , fun.y=mean, geom="line", size=1 ) +
    stat_summary( data = test_data2, aes(x=x, y=(1-value) , colour = as.factor(2), shape=as.factor(2)) , fun.y=mean, geom="point", size=3, pch=21, fill="white" ) +
    theme_bw(base_size = 14, base_family = "Palatino") + 
    theme(legend.key = element_blank() ) +
    expand_limits(x=c(0), y=c(0)) +
    facet_grid(distance ~ . ) 
  </code></pre>
  
  <p>How do I get rid of the grey background appearing behind <code>100</code> and <code>200</code>. </p>
  
  <p>I would like that to be white, without changing the rest of the theme I am using at the moment.</p>