Cumulative frequency by factor

Question

I have to find out the cumulative frequency, converted to percentage, of a continuous variable by factor. For example:

data <- data.frame(n = sample(1:12),
                d = seq(10, 120, by = 10),
                Site = rep(c("FirstSite", "SecondSite"), 6), 
                Plot = rep(c("Plot1", "Plot1", "Plot2", "Plot2"), 3)
                )

data <- with(data, data[order(Site,Plot),])
data <- transform(data, G = ((pi * (d/2)^2) * n) / 10000)

data
    n   d       Site  Plot           G
1   7  10  FirstSite Plot1  0.05497787
5   9  50  FirstSite Plot1  1.76714587
9  12  90  FirstSite Plot1  7.63407015
3  10  30  FirstSite Plot2  0.70685835
7   5  70  FirstSite Plot2  1.92422550
11  1 110  FirstSite Plot2  0.95033178
2   3  20 SecondSite Plot1  0.09424778
6   8  60 SecondSite Plot1  2.26194671
10  6 100 SecondSite Plot1  4.71238898
4   4  40 SecondSite Plot2  0.50265482
8   2  80 SecondSite Plot2  1.00530965
12 11 120 SecondSite Plot2 12.44070691

I need the cumulaive frequency of column G by factors Plot~Sitein order to plot a geom_step ggplot of G against d for each plot and site.
I have achieved to compute cumulative sum of G by factor by:

data.ss <- by(data[, "G"], data[,c("Plot", "Site")], function(x) cumsum(x))
# Gtot
(data.ss.tot <- sapply(ss, max))
[1]  9.456194  3.581416  7.068583 13.948671

Now I need to express each Plot G in the range [0..1] where 1 is Gtot for each Plot. I imagine I should divide G by its Plot Gtot, then apply a new cumsum to it. How to do it?
Please note that I have to plot this cumulative frequency against d not G itself, so it is not a proper ecdf.
Thank you.

Answer 1

I usually use ddply and transform to do this type of thing:

> data = ddply(data, c('Site', 'Plot'), transform, Gsum=cumsum(G), Gtot=sum(G))
> qplot(x=d, y=Gsum/Gtot, facets=Plot~Site, geom='step', data=data)