[R] Adding colored background area to a time series plot

[James Toll]

Hi,

I am trying to add a rectangular colored background area to a plot of a time series of relative price changes.  I believe that what I'm trying to do is very similar to the question and example given here:

http://www.mail-archive.com/r-help@stat.math.ethz.ch/msg73948.html
http://www.mayin.org/ajayshah/KB/R/html/g5.html

My problem/difference is that my time series looks like so:

> library(quantmod)
> getSymbols("^GSPC", from="2008-01-01")
> xtsPR <- diff(log(Ad(GSPC)))
> head(xtsPR)
                     GSPC.Adjusted
2008-01-02            NA
2008-01-03   0.000000000
2008-01-04  -0.024857950
2008-01-07   0.003218041
2008-01-08  -0.018522677
2008-01-09   0.013532064

So my dates along the x-axis are in the format "2008-01-02" rather than "2008", as in the example.

To keep things simple, I've simply been trying to plot the time series using the plot command:

plot(xtsPR)

and then adding either a polygon, or preferably, a rect:

rect(2009-01-02, -0.10, 2009-12-31, 0.10, border = 0, col = "blue")

But nothing is added to the plot when I try to add the rectangle, but I don't get an error message either.

My question is how do I specify my x coordinates to draw the polygon or rect, given that my scale is of the form "2008-01-02" rather than simply "2008"?

Additionally, do I have to use polygon, even if I'm trying to drawing a rectangle, as was the case in the examples linked above?

I'd like to be able to use this with chartSeries, rather than plot, but I thought it would be easiest to figure it out using plot first.  Clearly that hasn't helped me.

Thanks for any advice you can provide.


James