[R] how to add US map to basic plot

[YouLong Xia]

Dear my colleagues,

I am drawing US  land surafce enery and water flux figure, and  I can 
plot  baisc energy and water fulx figs using R. Now I am adding
US map  (mainly boundary for US and boundary of 48 states) to my basic 
plot. Could you please tell me to how do it (
plaese give me an example R source code if it possible)?  I am just a 
new begginer for R. Thanks,

Youlong