设置dot graphiz图片背景

graph demo { 

Server1 [fontname="SimSun", shape=circle, label=" http ", fillcolor="orange", style=filled] 
Server2 [fontname="SimSun", shape=circle, label=" www ", fillcolor="#ABACBA", style=filled] 
Server3 [fontname="SimSun", shape=circle, label=" dot ", fillcolor="yellow",style=filled] 
Server4 [fontname="SimSun", shape=circle, label=" com ", fillcolor="green",style=filled] 

Server1 -- Server2 
Server2 -- Server3 
Server3 -- Server4 

bgcolor="#ABABAB" 

}