So I am trying to make a stacked bar graph with bar width mapped to a variable; but I want the spacing between my bars to be constant.
Does anyone know how to make t
For a categorical or "discrete" scale - you can adjust the width, but it needs to be between 0 and 1. Your value.x's put it over 1, hence the overlap. You can use rescale, from the scales packages to adjust this quickly so that the within category width of the bar is representative of some other variable (in this case value.x)
install.packages("scales")
library(scales)
ggplot(dd,aes(x=variable,y=value.y,fill=Date)) +
geom_bar(aes(width=rescale(value.x,c(0.5,1))),stat="identity",position="stack")' +
coord_flip()
Play with rescaling for optimal "view" change 0.5 to 0.25... etc.

Personally, I think something like this is more informative:
ggplot(dd,aes(x=variable,y=value.y,fill=Date)) +
geom_bar(aes(width=rescale(value.x,c(0.2,1))),stat="identity") +
coord_flip() + facet_grid(~Date) + opts(legend.position="none")
