title: Change portion of the background in ggplot be to a different color
body: <p>Using the following data frame <em>d</em>: </p>
  
  <pre><code>day &lt;- gl(8,1,24,labels=c("Mon","Tues","Wed","Thurs","Fri","Sat","Sun","Avg"))
  day &lt;- factor(day, level=c("Mon","Tues","Wed","Thurs","Fri","Sat","Sun","Avg"))
  month&lt;-gl(3,8,24,labels=c("Jan","Feb","Mar"))
  month&lt;-factor(month,level=c("Jan","Feb","Mar"))
  snow&lt;-gl(1,24,labels=c("Y"))
  snow&lt;-factor(snow,levels=c("Y","N"))
  count &lt;- c(4,5,6,8,3,4,9,5.57,2,4,3,7,1,9,3,4.14,7,9,6,3,1,2,8,5.14)
  d &lt;- data.frame(day=day,count=count,month=month,snow=snow)
  </code></pre>
  
  <p>The background color behind the line graph is the same as the bar graph:</p>
  
  <pre><code>ggplot(data=d[d$day=="Avg",], aes(x=day , y=count, fill=month)) + 
  geom_bar(position = "dodge", width = 1.2, stat="identity") +
  geom_text(aes(label=month, x=day, y=count), position=position_dodge(width=1.2), vjust=-.6, size=3) + 
  geom_line(data=d[d$day!="Avg",], aes(x=day, y=count, group=month, colour=month)) + 
  facet_wrap(~snow,ncol=1,scales="free") + 
  scale_x_discrete(limits=levels(d$day))
  </code></pre>
  
  <p>Is it possible to change the background color on the portion behind the bar graph? </p>
  
  <p>Graph without partial background color change
  <a href="https://i.stack.imgur.com/8DzBx.png" rel="noreferrer"><img src="https://i.stack.imgur.com/8DzBx.png" alt="enter image description here"></a></p>