ggplotで複数のグラフを重ねる

データは以下のもの

>head(df)
 Ozone Solar.R Wind Temp Month Day       Date
1    41     190  7.4   67     5   1 2014-05-01
2    36     118  8.0   72     5   2 2014-05-02
3    12     149 12.6   74     5   3 2014-05-03
4    18     313 11.5   62     5   4 2014-05-04
5    NA      NA 14.3   56     5   5 2014-05-05
6    28      NA 14.9   66     5   6 2014-05-06

>ggplot(df,aes(x=Date))+geom_point(aes(y=Temp),color='red')+geom_point(aes(y=Wind),color='blue')+geom_point(aes(y=Solar.R),color='green3')+geom_point(aes(y=Ozone),color='purple')

色々ラベルを試行錯誤した結果、以下が良さそう
(aes内にcolorを記述、+ylab('')+labs(color="")を追加)

ggplot(df,aes(x=Date))+geom_point(aes(y=Temp,color='Temp'))+geom_point(aes(y=Wind,color='Wind'))+geom_point(aes(y=Solar.R,color='Solar.R'))+geom_point(aes(y=Ozone,color='Ozone'))+ylab("")+labs(color="")