[R] Stacked Histogram, multiple lines for dates of news stories?

[Simon Kiss]

Dear colleagues,
I have extracted the dates of several news stories from a newspaper  
data base to chart coverage trends of an issue over time. They are in  
a data frame that looks just like one generated by the reproducible  
code below.
I can already generate a histogram of the dates with various intervals  
(months, quarters, weeks years) using hist.Date.  However, there are  
two other things I'd like to do.
First, I'd like to either create a stacked histogram so that one could  
see whether one newspaper really pushed coverage of an issue at a  
certain point while others then followed later on in time.  Second, or  
alternatively, I would like to do a line graph of the same data for  
the different papers to represent the same trends.
I guess what I'm finding challenging is that I don't have counts of  
the number of stories on each day or in each week or in each month; I  
just have the dates themselves.  The date.Hist command was very useful  
in turning those into bins, but I'd like to push it a bit further and  
to a stacked histogram or a multiple line chart.
Can anyone suggest a way to go about doing this?

I should say, I played around in Hadley Wickham's ggplot package and  
looked at his website, and there is a way to render multiple lines  
here: http://had.co.nz/ggplot2/scale_date.html
but it was not clear to me how to plot just the dates or an index of  
the dates as I don't have a value for the y axis, other than the  
number of times a story was published in that time frame.

Regardless, I hope someone can suggest something.
Yours,
Simon J. Kiss

test=sample(1:3, 50, replace=TRUE)
test=as.factor(test)
levels(test)=c("Star", "Globe and Mail", "Post")
test2=ISOdatetime(sample(2004:2009, 50, replace=TRUE), sample(1:12,  
size=50, replace=TRUE), sample(1:30, 50, replace=TRUE), 0,0,0)
test2=as.Date(test2)
test_df=data.frame(test, test2)

*********************************
Simon J. Kiss, PhD
SSHRC and DAAD Post-Doctoral Fellow
John F. Kennedy Institute of North America Studies
Free University of Berlin
Lansstraße 7-9
14195 Berlin, Germany
Cell: +49 (0)1525-300-2812,
Web: http://www.jfki.fu-berlin.de/index.html