R语言小白想请教一下各位大神，正在用教材《数据统计分析及R语言编程》学习R语言，第59面“4.加图例参数”我把书上的代码一模一样地输到Rstudio里，不知道为什么运行总是报错 'legend' is of length 0代码：with(UG,{plot(height,weight);plot(height,weight,pch=as.numeric(region));legend(150,90,levels(region),pch=1:3,bty='n')})Error in legend(150, 90, levels(region), pch = 1:3, bty = "n") :   'legend' is of length 0In addition: Warning message:In plot.xy(xy, type, ...) : NAs introduced by coercion这是书本上的代码