[R] ggplot2 stacked bar - sum of values rather than count

[J Toll]

Hi,

I'm trying to create a stacked bar plot using ggplot2. Rather than
plotting the count of each of the 13 "Bar" factors on the Y axis, I
would like to represent the sum of the Values associated with each of
the 13 "Bar" factors. Is there a way to do that?  Given the following
data, that would obviously mean that there would be some negative sums
represented.  Here's a bit of example data along with the command I've
been using.

>library(ggplot2)
> x
         Value Bar Segment
1   1.10020075   1       1
2  -1.37734577   2       1
3   2.50702876   3       1
4   0.58737028   3       2
5   0.21106851   3       3
6  -2.50119261   4       1
7   1.34984831   5       1
8  -0.27556149   6       1
9  -1.54401647   6       2
10 -2.75975562   6       3
11 -0.09527123   6       4
12  1.36331646   7       1
13 -0.36051429   8       1
14  1.36790999   9       1
15  0.15064633   9       2
16  0.34022421   9       3
17 -0.64512970  10       1
18  0.83268199  11       1
19 -1.50117728  12       1
20  1.09004959  13       1
> qplot(factor(Bar), data = x, geom = "bar", fill = factor(Segment))

Thanks for any suggestions you might have.

James