[R] plotting points over a map using R

[李德磊]

Hello everyone,
I have no idea how to plotting points over a map using R, in which the colors of the points stand for the points' value. I'd like to get a map as shown in attachment picture which was plotted using matlab. Does anyone know how to do it using R?  Thank you very much for your help in advance.  
Kind regards,Dli 		 	   		  
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