title: Why setting transparency (alpha) for background is not working in ggplot?
body: <p>I'm facing some difficulties to set my background plot with some transparency.
  simply, whenever I set alpha to any value, same opaque background is returned.
  Although there is some posts regarding background highlighting such as </p>
  
  <p><a href="https://stackoverflow.com/questions/20194728/customize-background-to-highlight-ranges-of-data-in-ggplot">customize background to highlight ranges of data in ggplot</a></p>
  
  <p><a href="https://stackoverflow.com/questions/4733182/how-to-highlight-time-ranges-on-a-plot/4733446#4733446">How to highlight time ranges on a plot?</a></p>
  
  <p><a href="https://stackoverflow.com/questions/12133846/ggplot2-highlight-chart-area">ggplot2: highlight chart area</a></p>
  
  <p>But unfortunately, this issue is not discussed here or there.
  I would appreciate your comments</p>
  
  <p>Here is some data for demo</p>
  
  <pre><code>install.packages("ggplot2")
  </code></pre>
  
  <p>First , using <code>geom_rect</code></p>
  
  <pre><code>ggplot(mtcars, aes(x=wt, y=qsec)) + 
  geom_point(color="black",size=8) + 
  theme_bw() +
  geom_rect(aes(xmin=4,xmax=5,ymin=14,ymax=Inf),alpha=0.9,fill="green")
  </code></pre>
  
  <p>Playing with alpha wont help and ultimately, that is what I got</p>
  
  <p><a href="https://i.stack.imgur.com/RIh0m.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RIh0m.png" alt="data from mtcars"></a></p>
  
  <p>Using <code>annotate</code> function , I can not generate it also.</p>