title: ggplot geom_bar: stack and center
body: <p>I have a dataframe with shares in percent,columns representing different items, rows the respective share of interviewees answering in different categories. I want to produce a stacked barchart.</p>
  
  <pre><code>library(ggplot2)
  library(reshape2)
   test&lt;-data.frame(i1=c(16,40,26,18),
                 i2=c(17,46,27,10),
                 i3=c(23,43,24,10),
                 i4=c(19,25,20,36))
   rownames(test)&lt;-c("very i.","i.","less i.","not i.")
  
  test.m&lt;-melt(test)
  
  ggplot(test.m, aes(x=variable, y=value, fill=value)) + 
     geom_bar(position="stack", stat="identity")
  </code></pre>
  
  <p>Looks o.k., but I want <br>
  a) center the bars: positive answers (very i. and i) up and the bottom two classes (less i. and not i.) down. <br>
  b) each category (very i, i, less i, not i,) having the same colour.</p>
  
  <p>Any help would be much appreciated.</p>