geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?
plot5<-ggplot(df,aes(year,pollution))+geom_point()+geom_line()+labs(x="Year",y="Particulatematteremissions(tons)",title="MotorvehicleemissionsinBaltimore")得到错误:geom_path:Eachgroupconsistsofonlyoneobse