Hi, 


we all know that R is powerful in dealing with graphics. 

I am now trying to use R to show some geographic attributes for a
specific geographic area on a graph. For example, if I would like to
generate a graph showing the population densities (in terms of the
intensity of colors) for Illinois State by zip codes. I did some
research; and it seems that package "maptools" is relevant. But I have
the following couple of questions regarding to the application. 

1.  A State is grouped by several counties, and sometimes we could
further divide it into many areas by zip codes. Will there be any
differences in the shapefiles (.shp)? Do I need to get a specific
shapefile in order to graph on a zip code level? 

2. Where can I get this kind of shapefiles? 

3. There is another type of file (.dbf) which includes the attributes of
a geographic area. It seems I could put the population densities in it.
But then how could I relate it to the shapefile (.shp)? I mean how to
link a population density to its corresponding zip code area? 

4. Finally, how could I use intensity of colors to show the differences
in the population densities? 


I would greatly appreciate it if someone could provide me a simple
example. Thanks a lot! 


Ningwei


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