ggplot2 - Overlaying two/more graphs using R -

i have been referring stackoverflow posts , came query on gglot2 features.

the data set used code follows.

timestamp   data_1      data_2      data_3       15:11:37.474    99.791028   0.312498    99.47853     15:16:22.373    99.791028   0.729162    99.061866    15:21:37.424    99.791028   0.104166    99.686862    15:31:52.475    88.02027            90.520254    15:42:07.157    99.99936    0.208332    99.791028    15:43:22.279    99.99936    0.52083     99.47853     15:45:37.673    99.686862   0       99.686862    15:52:52.872    99.686862   0.729162    98.9577   p1<- ggplot(df, aes(timestamp, data_1,group=1)) + geom_point() + geom_point(data = df[df$data_1 > 80,], pch = 21, fill = na, size = 4, colour = "red", stroke = 1) + geom_line() p2<-ggplot(df, aes(timestamp, data_3,group=1)) + geom_point() + geom_point(data = df[df$data_3 > 70,], pch = 21, fill = na, size = 4, colour = "blue", stroke = 1) + geom_line() 

i trying overlay p1 , p2 same x , y axis using print(p1+p2). but, came below error.

error in p + o : non-numeric argument binary operator in addition: warning message: incompatible methods ("+.gg", "ops.data.frame") "+"  

what these graphs is; circle data points based on conditions , display 2 graphs in same region.

try

p1<- ggplot(df, aes(timestamp, data_1,group=1)) + geom_point(pch = 21, fill = na, size = 4, colour = "red", stroke = 1)+geom_point()+geom_line(colour = "red") p2 <- p1+geom_line(data=df, aes(timestamp, data_3),colour="blue")+   geom_point(data=df, aes(timestamp, data_3,group=data_3),colour="blue",pch = 21, size = 4, stroke = 1,fill= na)+geom_point(data=df, aes(timestamp, data_3,group=data_3))   print(p2)