[R-sig-Geo] Trying to plot raster variance over time

[Taylor Turner]

> Hello R-masters, 
> I am relatively new to R and I am even newer to its use in GIS applications so please bear with me. I am currently using all available GIMMS of Africa (from '81 to '06) and I plan on plotting the mean NDVI variance of each month over the years (1981-2006) of several smaller ranges that I have cropped out and then stacked. Each stack consists of 600 rasters - 24 semimonthly GIMMS for 25 years. Here is my problem: after reading various examples it seems that I first need to assign a z value (using setZ I presume). How can I categorize the 600 rasters into months and assign z values? 
> 
> Thanks,
> T