How can I make consistent-width plots in ggplot (with legends)?

I've got a few different categories that I want to plot. These are different categories, each with their own set of labels, but which makes sense to group together in the document. The following gives some simple stacked bar chart examples:

df <- data.frame(x=c("a", "b", "c"),
                 y=c("happy", "sad", "ambivalent about life"))
ggplot(df, aes(x=factor(0), fill=x)) + geom_bar()
ggplot(df, aes(x=factor(0), fill=y)) + geom_bar()

The problem is that with different labels, the legends have different widths, which means the plots have different widths, leading to things looking a bit goofy if I make a table or \subfigure elements. How can I fix this?

Is there a way to explicitly set the width (absolute or relative) of either the plot or the legend?