> states <- as.data.frame(state.x77[,c("Murder","Population","Illiteracy","Income","Frost")])
> fit <-  lm (Murder ~ Population + Illiteracy + Income + Frost,data = states)
> avPlots(fit,ask=FALSE,onepage=TRUE,id.method="identify")
上段代码为什么得不到变量添加图？该怎么修改？