[R] ggplot: colours to geom_segments

[John Kane]

Just as an exercise I am tying to add colours to a geom_segment command. I can get one colour but not a sequence of colours.  
Can anyone suggest how I can get the green lines in the plot below to be different colours?  I thought I could use a palatte of colours but that did not seem to work.

Thanks

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library(ggplot2)

xx <- structure(list(Food = structure(c(2L, 1L, 3L, 4L), .Label = c("Bread", 
"Milk", "Potatoes", "Rice"), class = "factor"), Expense = c(25, 
49, 34, 15)), .Names = c("Food", "Expense"), row.names = c(NA, 
-4L), class = "data.frame")

p <- ggplot(xx, aes(x = 0, xend = Expense,  y = Food, yend = Food))
pa <- p + geom_point(aes(Expense, Food)) + 
       geom_segment(colour="darkgreen") + 
       xlab("Food") + geom_vline(xintercept=40, colour='red') 
pa