Dividing lines between sections of bar plot with ggplot2 in r

Question

I am using qplot to create a stacked bar plot and would like to place a white line between the sections of each bar as the blues seem to almost blend together. I do not want to change my existing colour scheme to resolve the problem. Any ideas?

library(ggplot2)
qplot(carat, data = diamonds, geom = "histogram", fill = color)

Answer 1

Add the argument colour="white" to create a white outline:

ggplot(mtcars, aes(factor(cyl), fill=am, group=am)) + geom_bar(colour="white")

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Here is a workaround to remove the diagonal line from the legend (inspired by a posting on ggplot mailing list ). The idea is to plot the geom_bar twice, once suppressing the colours:

ggplot(mtcars, aes(factor(cyl), fill=am, group=am)) + 
  geom_bar() +
  geom_bar(colour="white", show_guide=FALSE)