How to plot three categorical variables and one continuous variable using ggplot2

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This post shows how to produce a plot involving three categorical variablesand one continuous variable using ggplot2 in R.

The following code is also available as a gist on github.

1. Create Data

First, let's load ggplot2 and create some data to work with:

library(ggplot2)set.seed(4444)Data <- expand.grid(group=c("Apples", "Bananas", "Carrots", "Durians",             "Eggplants"),            year=c("2000", "2001", "2002"),            quality=c("Grade A", "Grade B", "Grade C", "Grade D",             "Grade E"))Group.Weight <- data.frame(    group=c("Apples", "Bananas", "Carrots", "Durians", "Eggplants"),    group.weight=c(1,1,-1,0.5, 0))Quality.Weight <- data.frame(    quality=c("Grade A", "Grade B", "Grade C", "Grade D", "Grade E"),    quality.weight = c(1,0.5,0,-0.5,-1))Data <- merge(Data, Group.Weight)Data <- merge(Data, Quality.Weight)Data$score <- Data$group.weight + Data$quality.weight +     rnorm(nrow(Data), 0, 0.2)Data$proportion.tasty <- exp(Data$score)/(1 + exp(Data$score))

2. Produce Plot

And here's the code to produce the plot.

ggplot(data=Data,        aes(x=factor(year), y=proportion.tasty,            group=group,           shape=group,           color=group)) +                geom_line() +                geom_point() +               opts(title =                "Proportion Tasty by Year, Quality, and Group") +               scale_x_discrete("Year") +               scale_y_continuous("Proportion Tasty") +         facet_grid(.~quality )

And here's what it looks like: