Choropleth (等值线图，等高线图) - useful to visualize spatial trends

Here, websites shows how to plot Choropleth:

http://rstats.posterous.com/choropleths-made-easy

http://offensivepolitics.net/blog/category/rstats/

http://media.apps.chicagotribune.com/election-maps/mayor-2011/index.html#41.85358024690839,-87.63826971435547,11