Set specific fill colors in ggplot2 by sign

Question

Hi wanted to adjust the following chart so that the values below zero are filled in red and the ones above are in dark blue. How can I do this with ggplot2?

   mydata = structure(list(Mealtime = "Breakfast", Food = "Rashers", `2002` = 9.12, 
                            `2003` = 9.5, `2004` = 2.04, `2005` = -20.72, `2006` = 18.37, 
                            `2007` = 91.19, `2008` = 94.83, `2009` = 191.96, `2010` = -125.3, 
                            `2011` = -18.56, `2012` = 63.85), .Names = c("Mealtime", "Food", "2002", "2003", "2004", "2005", "2006", "2007", "2008","2009", "2010", "2011", "2012"), row.names = 1L, class = "data.frame")
x=ggplot(mydata) +
  aes(x=colnames(mydata)[3:13],y=as.numeric(mydata[1,3:13]),fill=sign(as.numeric(mydata[1,3:13]))) +
  geom_bar(stat='identity') + guides(fill=F)
print(x)

Answer 1

The way you structure your data is not how it should be in ggplot2:

require(reshape)
mydata2 = melt(mydata)

Basic barplot:

ggplot(mydata2, aes(x = variable, y = value)) + geom_bar()

The trick now is to add an additional variable which specifies if the value is negative or postive:

mydata2[["sign"]] = ifelse(mydata2[["value"]] >= 0, "positive", "negative")

..and use that in the call to ggplot2 (combined with scale_fill_* for the colors):

ggplot(mydata2, aes(x = variable, y = value, fill = sign)) + geom_bar() + 
  scale_fill_manual(values = c("positive" = "darkblue", "negative" = "red"))