r - ggplot geom_bar: stack and center

Question

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.

library(ggplot2)
library(reshape2)
 test<-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)<-c("very i.","i.","less i.","not i.")

test.m<-melt(test)

ggplot(test.m, aes(x=variable, y=value, fill=value)) + 
   geom_bar(position="stack", stat="identity")

Looks o.k., but I want
a) center the bars: positive answers (very i. and i) up and the bottom two classes (less i. and not i.) down.
b) each category (very i, i, less i, not i,) having the same colour.

Any help would be much appreciated.

Answer 1

It is better to use the category names as a separator instead of row names:

test$category <- factor(c(3,4,2,1), labels=c("very i.","i.","less i.","not i."))

(The ordering of the factor levels is done with repect to the stacked barplot (lowest: not i., highest: very i.).

test.m <- melt(test)

To answer your questions:

1. Stacked barplots do not work well if some values are above and others are below zero. Hence, two separate barplots are created (one with negative values, one with positive values).
2. The new column category is used for the fill parameter to map each category to a different colour.

The complete code:

ggplot(test.m, aes(x=variable, fill=category)) + 
      geom_bar(data = subset(test.m, category %in% c("less i.","not i.")),
               aes(y = -value), position="stack", stat="identity") +
      geom_bar(data = subset(test.m, !category %in% c("less i.","not i.")), 
               aes(y = value), position="stack", stat="identity")