title: Adding Shaded region to histogram between quintiles
body: <p>All,</p>
  <p>I have a chart that has 2 histograms in which I also plotted lines representing the 20th, 40th, 60th and 80th percentiles,  the code below reproduces a similar chart with dummy data</p>
  <pre><code>data &lt;- rbind(data.frame(x=rnorm(1000,0,1),g=&quot;one&quot;),data.frame(x=rnorm(1000,0.2,1.5),g=&quot;two&quot;))
  q1 = melt(ddply(melt(data,id.vars=&quot;g&quot;),.(g),summarise,q20=quantile(value,.2,na.rm=T),
             q40=quantile(value,.4,na.rm=T),q60=quantile(value,.6,na.rm=T),q80=quantile(value,.8,na.rm=T)))
  ggplot(data,aes(x=x,fill=g))+
    geom_vline(data=q1,aes(xintercept=value,group=variable),linetype=2,color=&quot;black&quot;)+
    #stat_bin(aes(y=..count../sum(..count..)),binwidth=0.1,alpha=0.4,geom=&quot;bar&quot;)+
    geom_histogram(aes(y=..count../sum(..count..)),alpha=0.75,binwidth=0.1)+facet_grid(g~.)+theme_bw()+coord_cartesian(xlim=c(-3,3))
  </code></pre>
  <p><img src="https://i.stack.imgur.com/8AaZu.png" alt="enter image description here" /></p>
  <p>I want to add a shaded region (maybe in gray) between the 40 and 60th lines to show the middle quintile - I would like for the region to be data driven (I am using dataframe q1 which is data derived and that is acceptable, I just do not want to have to enter value manually)</p>
  <p>How can this be accomplished? I have not been able to do it</p>
  <p>thanks for all the help</p>