boxplot数值变量和分类变量关系画图报错
win10，Rstudio 1.0.143，输入boxplot(wind~month, airquality, xlab="month", ylab="wind speed")报错：Error in eval(predvars, data, env) : object 'wind' not found，但是输入boxplot(airquality$Wind~airquality$Month,xlab="month",ylab="wind speed")正常显示，为什么？
