[R] map at fips level using multiple variables

[bby2103 at columbia.edu]

Hi, I just started playing with county FIPS feature in maps package  
which allows geospatial visualization of variables on US county level.  
Pretty cool.

I did some search but couldn't find answer to this question--how can I  
map more than 2 variables on US map? For example, you can map by the  
breakdown of income or family size. How do you further breakdown based  
on the values of both variables and show them on the county FIPS level?

Thanks in advance!

Bonnie Yuan