[R] Exporting a Graph that has lines()

[R_Learner]

I'm trying to export a graph (any form is good). I'm trying to export one
that uses plot() then lines(). however, it seems that the jpeg format and
the pdf format only takes the first line plot, and doesn't even consider the
additional plots. My code is the following:
	
output_file<-paste(filez,".pdf",sep="")
pdf(output_file)
plot((x_data) ~ (y_data), main = filez)
lines (x,z,col="red")
dev.off()

The pdf only comes out with the graph of the plot, and not of the lines.
Does anyone know how to export multiple graph items on one plot?

Thanks!
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