title: Modify alpha values by group in ggplot2
body: <p>I would like to change alpha scale, now I think it's from 0 to 1. 
  I would like to make it from 0.2 to 0.5 or so..'cause in this way we can't see Event filling colours very well..M"  is too dark and M1 too light..</p>
  
  <p>Here's the database infos:</p>
  
  <pre><code>&gt; str(st2)
  
  'data.frame':   24 obs. of  6 variables:
   $ Outcome   : Factor w/ 4 levels "LGiT","LGuT",..: 3 3 3 1 1 1 2 2 2 4 ...
   $ Event     : Factor w/ 3 levels "Event 1","Event 2",..: 1 2 3 1 2 3 1 2 3 1 ...
   $ Percentage: num  10.31 8.25 81.44 12.77 15.32 ...
   $ Method    : Factor w/ 2 levels "M1","M2": 1 1 1 1 1 1 1 1 1 1 ...
   $ Cohort    : Factor w/ 1 level "Cohort 1": 1 1 1 1 1 1 1 1 1 1 ...
   $ label     : chr  "10%" "8%" "81%" "13%" ...
   $ sum       : chr  "100.00%" "100.00%" "100.00%" "100.00%" ...
  </code></pre>
  
  <p>Here's the plot code:</p>
  
  <pre><code>p7&lt;-ggplot(st2, aes(x = factor(st2$Outcome:st2$Method), y = st2$Percentage,  
                  fill = st2$Event, group=st2$Event)) +
    geom_bar(aes(alpha=st2$Method),position = position_stack(), stat = "identity", width = .7, colour="black") +
    geom_text(aes(label = st2$label), position = position_stack(vjust = 0.5),size = 4) + coord_flip() 
  
  
  p8&lt;-p7+labs(x="Outcome", y="Percentage Values", fill="Legend")+ scale_fill_grey(start=1, end=0.8)+ theme(axis.text=element_text(size=15), axis.title=element_text(size=17, face="bold"), legend.title=element_text(size=13), legend.text=element_text(size=12))
  
  
  p8+theme_bw() + theme(panel.border = element_blank(), panel.grid.major = element_blank(), panel.grid.minor = element_blank(), axis.line = element_line(colour = "black"))
  </code></pre>
  
  <p>Here is my plot:</p>
  
  <p><a href="https://i.stack.imgur.com/lIJW1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lIJW1.png" alt="enter image description here"></a>
  I've tried by adding</p>
  
  <pre><code>scale_alpha(limits=c(0.2,0.5))
  </code></pre>
  
  <p>but it doesn't work: Error: Discrete value supplied to continuous scale</p>
  
  <p>Thank you!</p>